feat: change sidebar color, improve YMD theme

[Feichtmeier]

This brightens up the sidebar for the dark theme a bit and darkens up the sidebar for the light theme a bit. This improves the visual focus on the content on the right (or left)
Plus this also adapts our looks to libadwaita

Possible variants:

Default:

light:

default:

[Feichtmeier]

those variants are not inherited by some enum or so because they are too complex for
appbar | appbar
list | detailpage
combos

but they are possible by setting the background colors of YaruWindowTitleBars and by wrapping the YMD in a YMD with copy with and changing the defaults

[Jupi007]

I like it, LGFM 👍

[Feichtmeier]

@jpnurmi any veto? 😄

[jpnurmi]

LGTM 👍

p.s. navi-rail too?

[Feichtmeier]

LGTM 👍

p.s. navi-rail too?

yeah I should, right?
in this PR or another?
Navirail in material defaults to colorscheme.surface
I wonder if we could rather let yaru navi rail use the material theme? because it is so close to it

[Jupi007]

I wonder if we could rather let yaru navi rail use the material theme? because it is so close to it

IMO it is not a good idea.

[jpnurmi]

A separate PR is fine, of course, if you decide to apply similar changes. :)

[Feichtmeier]

I wonder if we could rather let yaru navi rail use the material theme? because it is so close to it

IMO it is not a good idea.

okay then I apply those changes to YaruNavigationPageThemeData in a second PR