Add less harcoded style.css to easy use GTK3 theming

In system/share/ui we have 3 files:

• style.css [1]
• adwaita.css [2]
• adwaita-dark.css [2]

[1] https://etherpad.gnome.org/p/Inkscape_1.0-style.css
[2] https://etherpad.gnome.org/p/Inkscape_1.0-AdwaitaANDAdwaita-dark.css

This files and more can be added to user/share/ui overriding system wide.

style.css must be theme independent, we must not add clases that can render bad with other GTK3 themes. Safe things to add here:

• Thinks in UI that affect globaly theme independent.
• Change of colors using @theme_ colors (there are some problems with gradients specialy with background-image property we need to use with care.
• Widgets created for inkscape only that can result on umpredictable results if not full defined here.

Adwaita.css and his copy Adwaita-dark.css: Here we put adwaita specific fix. it works for any theme if we create a CSS with theme_name-dot-css (you can read in preferences.xml if have doubts) and can be put it in user or system UI folders. Anyway we don't claim for making it. If we make a style.css very isolated probably users don't want to do.

Old issue

Just a atempt to make style.css less harcoded as posibol to allow teming by the user.
I just open this etherpad to collaborative improve it:

https://etherpad.gnome.org/p/Inkscape_1.0-style.css