Improve dark mode colors

[mochaaP]

Is your feature request related to a problem? Please describe.
Currently the playfield (both key and square) doesn't have a dark color when gtk_application_prefer_dark_theme is true, as illustrated below:

This leads to an unpleasant high contrast when dark mode is activated.

Describe the solution you'd like

As other GTK/native elementary OS applications choose to use a darker color for inputs, a lighter color for buttons, we can also follow their behavior.

• Squares should have a darker background than base (it's supposed to be deeper when considering a real-life game board)
• Keys should have a brighter background than base

Describe alternatives you've considered
Use native GTK components and color with semantic color names (gdk_color_parse)

[avojak]

Will be addressed as part of updating to GTK 4, which will use more CSS instead of very specific images for each tile state.

[avojak]

Resolved with #39