feat: Color mixing for Monet generation

[daudix]

Feature description

As discussed in Matrix, we need a way to mix two colors in Monet, one of them with small opacity, e.g mix @window_bg_color with full opacity and @accent_bg_color with 0.4 opacity, this is needed for fine-tuning coloring of headerbar, views, dialogs, popeovers and etc

Show illustration

Edit: there is simpler way, putting this to any color prompt mixes two given colors, now it's just a matter of putting this snippet in required prompts on Monet generation

alpha(mix(@accent_bg_color, @window_bg_color, 0.8), 1)

Edit 2: not sure if this will work for GTK3, so generating hex is likely a better solution

Additional Context

No response

Did you read the Code of Conduct?

• I have read the Code of Conduct.