Colors: Darken gray for borders/separators; "soften" button colors

Darken $lightgray to make separations more pronounced. Add new color,
$lightergray, a 5% lighter variation of $lightgray, for buttons on sidebar
to prevent them from appearing too bright.

Take $black down a notch to decrease the black/white contrast on sidebar
buttons.
Sidebar: Change search box and submit buttons to use $content colors

For stylistic consistency, remove $box-background-highlight and treat the
search box's "advanced" text and the submission buttons as regular
content.
Color: Soften border

Change $content-border to be a lighter gray for less contrast.
Link flairs: Set absolute width

Set all link flairs to 80px to avoid the issue where post titles would not
line up after being flaired.
Bump to v0.1-beta

Ready for beta testing in /r/writing.