feat: make dark theme darker

[Feichtmeier]

This PR adds a darker version of Jet (#2b2b2b) which is "Dark Jet" #252525 and applies it as the material theme's background color.

There are two main colours for things in the background in material themes

• surface, used for sidebars, appbars, bottombars, etc
• background, used for the scaffold background color mainly

Reasons why I think this is better:

• the content of windows can now have a better contrast if wanted, previously there was only 1 colour for backgrounds
• in comparison to GTK4 windows flutter windows look less bright overall (in average)

Before	After

Please everyone involved have a look and say what you think @Jupi007 @jpnurmi @madsrh @elioqoshi

Note: the textfields now are light against dark and before dark against light, this could need fixing if you want

Fixes #234

[Jupi007]

I really like it, there's a real contrast between the headerbar and the window body 👍

[elioqoshi]

This is a good improvement +1

[Feichtmeier]

@jpnurmi since we've got some design thumbs up by Paul, mads and Elio and also me could you see if the code is okay for you (review)? Or ofc see if you have veto against the look