For themes with primarily light text colors and dark backgrounds the
current almost-white background/black text tooltips look a bit out of
place. I've changed them to what I believe are sensible colors but theme
authors are of course free to tweak further.
Change #2811 made window title stripes and window title shadow themable,
but it used the same stripe and shadow color for all window modes.
This is fine for the new 'basalt' theme which uses the same color
in all four window modes, but it changed the default theme so that
background windows had brown stripes and a brown shadow.
Instead, make the title stripe and title shadow themable per window mode,
and change the default theme to restore the colors it had before
change #2811: The title stripe color is the same as Border1 for all
window modes, and the title shadow is the same as the title stripe
darkened by 0.6.
The theming system can now control title bar height, title button
size, title stripe color and the title text shadow color.
The implemented theme metrics system could be later extended to LibGUI
to allow themes to change widget padding, border width, etc.
Nord (specifically Polar Night) is a popular soft dark blue/grey theme.
Personally, I find it to be a very nice middle-ground between the contrast of the
dark and light (default) theme.

Add "Link", "ActiveLink" and "VisitedLink" colors to the system theme
definition, and implement support for them in LibHTML.
Note that <body link="foo" alink="bar" vlink="baz"> takes precedence
over the system colors. Author style also takes precedence, since we
only fetch the system color in case the CSS color is -libhtml-link.
Color themes are loaded from .ini files in /res/themes/
The theme can be switched from the "Themes" section in the system menu.
The basic mechanism is that WindowServer broadcasts a SharedBuffer with
all of the color values of the current theme. Clients receive this with
the response to their initial WindowServer::Greet handshake.
When the theme is changed, WindowServer tells everyone by sending out
an UpdateSystemTheme message with a new SharedBuffer to use.
This does feel somewhat bloated somehow, but I'm sure we can iterate on
it over time and improve things.
To get one of the theme colors, use the Color(SystemColor) constructor:
painter.fill_rect(rect, SystemColor::HoverHighlight);
Some things don't work 100% right without a reboot. Specifically, when
constructing a GWidget, it will set its own background and foreground
colors based on the current SystemColor::Window and SystemColor::Text.
The widget is then stuck with these values, and they don't update on
system theme change, only on app restart.
All in all though, this is pretty cool. Merry Christmas! :^)