gtk-style: Remove .boxes-machine-name-label

This class was introduced in ea48c29e and its usage was removed in
2526a28c.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
gtk-style: Remove unneeded hardcoded colors

The inherited values look almost the same and provide better
maintainability due to less hardcoded colors and less code in general.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
gtk-style: Replace hardcoded colors by theme colors

The colors from the theme look almost the same while providing better
maintainability due to less hardcoded colors and less code in general.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
gtk-style: Replace .boxes-continue by .suggested-action

The .suggested-action class is available from the theme. There is no
need to maintain an extra clone of this class.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
util-app: Remove unused get_boxes_bg_color()

This function was introduced back in b60fccda and its usage was removed
in 6155dc10.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
wizard: Drop use of .boxes-bg

.boxes-bg defines that the background color is transparent. This
property is overwritten by the .wizard class and therefore the use of
this class is unneeded here.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
gtk-style: Rename .boxes-bg to .transparent-bg

The new name describes way better that the class just makes the
background transparent.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
gtk-style: Rename .wizard to .content-bg

This class describes the background for any content area and should have
an appropriate name. With the new name this class will later be used to
describe other content backgrounds outside the wizard.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
gtk-style: Replace unneeded classes by .content-bg

The .content-bg class does exactly the same as these classes so we can
use this everywhere.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
content-bg: Use theme color instead of image

This is a minor visual change while it allows Boxes to adapt to theme
changes and especially makes themes like the high contrast accessibility
theme work.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
sidebar: Use theme color instead of image

This is a minor visual change while it allows Boxes to adapt to theme
changes and especially makes themes like the high contrast accessibility
theme work.

https://bugzilla.gnome.org/show_bug.cgi?id=731952
BoxesMiniGraph: Replace hardcoded colors by theme colors

The BoxesMiniGraph will now invert colors with the theme if needed.

This is a minor visual change while it allows Boxes to adapt to theme
changes and especially makes themes like the high contrast accessibility
theme work.

https://bugzilla.gnome.org/show_bug.cgi?id=731952