[Android Themes] Ticklefish:
Personally, I wouldn't worry about editing styles.xml. I'd just add a textSize attribute to the date line in the XML..

<?xml version="1.0" encoding="utf-8"?>
<com.android.systemui.statusbar.phone.StatusBarHeaderView android:layout_gravity="center_horizontal" android:id="@id/header" android:background="@drawable/notification_header_bg" android:focusable="true" android:clickable="true" android:descendantFocusability="afterDescendants" android:layout_width="@dimen/notification_panel_width" android:layout_height="@dimen/status_bar_header_height" android:baselineAligned="false" android:paddingStart="@dimen/notification_side_padding" android:paddingEnd="@dimen/notification_side_padding"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:systemui="http://schemas.android.com/apk/res/com.android.systemui">
<ImageButton android:id="@id/settings_button2" android:visibility="gone" android:layout_width="48.0dip" android:layout_height="@dimen/status_bar_header_height" android:src="@drawable/ic_notify_edit_normal" style="@android:style/Widget.Material.Button.Borderless" />
<com.android.systemui.statusbar.phone.MultiUserSwitch android:id="@id/multi_user_switch" android:background="@drawable/ripple_drawable" android:layout_width="wrap_content" android:layout_height="fill_parent" android:scaleType="center" android:layout_marginStart="@dimen/button_holder_margin_left" android:layout_marginEnd="@dimen/button_holder_margin_right" android:layout_toStartOf="@id/edit_button_holder">
<ImageView android:layout_gravity="center" android:id="@id/multi_user_avatar" android:layout_width="24.0dip" android:layout_height="24.0dip" android:scaleType="centerInside" />
</com.android.systemui.statusbar.phone.MultiUserSwitch>
<FrameLayout android:id="@id/edit_button_holder" android:layout_width="wrap_content" android:layout_height="@dimen/button_holder_width" android:layout_toStartOf="@id/expanded_divider_3">
<TextView android:textSize="14.0dip" android:textColor="@color/status_bar_header_text_icon_tint_color" android:gravity="center" android:layout_gravity="center" android:id="@id/qs_edit_button" android:background="@drawable/ripple_drawable" android:paddingTop="@dimen/button_holder_margin_top" android:paddingBottom="@dimen/button_holder_margin_bottom" android:focusable="true" android:visibility="gone" android:clickable="true" android:layout_width="@dimen/button_edit_text_width" android:layout_height="fill_parent" android:text="@string/qs_edit_setting_edit_button" android:singleLine="true" android:layout_marginStart="@dimen/button_holder_margin_left" android:layout_marginEnd="@dimen/edit_text_margin_right" />
<ImageView android:id="@id/basic_settings_button" android:background="@drawable/ripple_drawable" android:paddingTop="@dimen/button_holder_margin_top" android:paddingBottom="@dimen/button_holder_margin_bottom" android:focusable="true" android:nextFocusLeft="@id/date_timeView" android:nextFocusRight="@id/settings_button" android:layout_width="fill_parent" android:layout_height="fill_parent" android:src="@drawable/ic_notify_quicksettings" android:scaleType="center" android:layout_marginStart="@dimen/button_holder_margin_left" android:layout_marginEnd="@dimen/button_holder_margin_right" />
<ImageView android:id="@id/settings_edit_button" android:background="@drawable/ripple_drawable" android:paddingTop="@dimen/button_holder_margin_top" android:paddingBottom="@dimen/button_holder_margin_bottom" android:focusable="true" android:visibility="gone" android:nextFocusLeft="@id/date_timeView" android:nextFocusRight="@id/notification_button" android:layout_width="fill_parent" android:layout_height="fill_parent" android:src="@drawable/ic_notify_edit_normal" android:scaleType="center" android:layout_marginStart="@dimen/button_holder_margin_left" android:layout_marginEnd="@dimen/button_holder_margin_right" />
</FrameLayout>
<ImageView android:id="@id/expanded_divider_3" android:background="@color/quickpanel_button_divider_color" android:visibility="gone" android:layout_width="1.0dip" android:layout_height="@dimen/button_holder_divider_height" android:layout_marginTop="@dimen/button_holder_divider_margin_top" android:layout_marginBottom="@dimen/button_holder_divider_margin_bottom" android:layout_marginEnd="@dimen/edit_text_margin_right" android:layout_toStartOf="@id/settings_button_holder" />
<FrameLayout android:id="@id/settings_button_holder" android:layout_width="@dimen/button_holder_width" android:layout_height="@dimen/button_holder_width" android:layout_alignParentEnd="true">
<ImageView android:id="@id/settings_button" android:background="@drawable/ripple_drawable" android:paddingTop="@dimen/button_holder_margin_top" android:paddingBottom="@dimen/button_holder_margin_bottom" android:focusable="true" android:nextFocusLeft="@id/basic_settings_button" android:nextFocusRight="@id/date_timeView" android:layout_width="fill_parent" android:layout_height="fill_parent" android:src="@drawable/ic_notify_settings" android:scaleType="center" android:tint="@color/status_bar_header_text_icon_tint_color" android:layout_marginStart="@dimen/button_holder_margin_left" android:layout_marginEnd="@dimen/button_holder_margin_right" />
<ImageView android:id="@id/notification_button" android:background="@drawable/ripple_drawable" android:paddingTop="@dimen/button_holder_margin_top" android:paddingBottom="@dimen/button_holder_margin_bottom" android:focusable="true" android:visibility="gone" android:nextFocusLeft="@id/settings_edit_button" android:nextFocusRight="@id/date_timeView" android:layout_width="fill_parent" android:layout_height="fill_parent" android:src="@drawable/ic_notifications" android:scaleType="center" android:layout_marginStart="@dimen/button_holder_margin_left" android:layout_marginEnd="@dimen/button_holder_margin_right" />
</FrameLayout>
<LinearLayout android:id="@id/system_icons_super_container" android:background="@drawable/ripple_drawable" android:visibility="gone" android:layout_width="wrap_content" android:layout_height="@dimen/status_bar_header_height" android:layout_alignWithParentIfMissing="true" android:paddingEnd="4.0dip" android:layout_marginStart="16.0dip" android:layout_toStartOf="@id/multi_user_switch">
<FrameLayout android:layout_gravity="center_vertical" android:id="@id/system_icons_container" android:layout_width="wrap_content" android:layout_height="@dimen/status_bar_height">
<include layout="@layout/system_icons" />
</FrameLayout>
<TextView android:textSize="@dimen/battery_level_text_size" android:textColor="#ffffffff" android:layout_gravity="center_vertical" android:id="@id/battery_level" android:layout_width="wrap_content" android:layout_height="wrap_content" android:importantForAccessibility="noHideDescendants" android:paddingEnd="@dimen/battery_level_padding_end" android:layout_marginStart="@dimen/header_battery_margin_expanded" />
</LinearLayout>
<TextView android:textAppearance="@style/TextAppearance.StatusBar.Expanded.EmergencyCallsOnly" android:gravity="center_vertical" android:id="@id/header_emergency_calls_only" android:visibility="gone" android:layout_width="wrap_content" android:layout_height="@dimen/status_bar_header_height" android:text="@android:string/emergency_calls_only" android:singleLine="true" android:paddingStart="16.0dip" android:paddingEnd="16.0dip" android:layout_toStartOf="@id/system_icons_super_container" android:layout_alignParentStart="true" />
<RelativeLayout android:id="@id/date_timeView" android:background="@drawable/ripple_drawable" android:focusable="true" android:layout_width="wrap_content" android:layout_height="wrap_content">
<com.android.systemui.statusbar.policy.DateView [color="red"]android:textSize="20.0dip"[/color] android:textAppearance="@style/TextAppearance.StatusBar.Expanded.Date" android:id="@id/date_collapsed" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_marginTop="@dimen/notification_panel_header_date_margin_top" android:singleLine="true" android:layout_marginStart="@dimen/clock_expanded_margin_start" />
<com.android.systemui.statusbar.policy.DateView [color="red"]android:textSize="20.0dip"[/color] android:textAppearance="@style/TextAppearance.StatusBar.Expanded.Date" android:id="@id/date_expanded" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_marginTop="@dimen/notification_panel_header_date_expanded_margin_top" android:singleLine="true" android:layout_alignParentLeft="true" android:layout_marginStart="@dimen/clock_expanded_margin_start" systemui:datePattern="@string/abbrev_wday_month_day_no_year_alarm" />
</RelativeLayout>
<include android:id="@id/qs_detail_header" android:layout_width="fill_parent" android:layout_height="wrap_content" android:layout_alignParentBottom="true" layout="@layout/qs_detail_header" />
<ImageView android:id="@id/qs_detail_header_progress" android:background="@color/qs_detail_progress_track" android:layout_width="fill_parent" android:layout_height="wrap_content" android:src="@drawable/indeterminate_anim" android:layout_alignParentBottom="true" android:alpha="0.0" />
<TextView android:textSize="11.0dip" android:textStyle="bold" android:textColor="#ff00a040" android:layout_gravity="center_vertical" android:id="@id/header_debug_info" android:padding="2.0dip" android:visibility="invisible" android:layout_width="wrap_content" android:layout_height="wrap_content" android:fontFamily="sans-serif-condensed" />
<TextView android:textSize="20.0dip" android:textColor="@color/qs_edit_available_btn_color" android:id="@id/notificaiton_edit_panel_back_button" android:background="@drawable/qs_background_primary" android:paddingLeft="14.0dip" android:paddingTop="14.0dip" android:focusable="true" android:visibility="gone" android:clickable="true" android:layout_width="fill_parent" android:layout_height="fill_parent" android:text="@string/qs_edit_settings_title" android:drawableLeft="@drawable/quickpanel_edt_ic_back" android:drawablePadding="16.0dip" android:selectable="false" />
</com.android.systemui.statusbar.phone.StatusBarHeaderView>

That way, you're not affecting anything else that uses that particular style..