Display.css: Improve look with dark style

[cassidyjames]

This makes the display color slightly transparent which lowers the contrast with the background a bit. The result is darker displays with dark style, and slightly lighter displays with the light style. While I was here, I also updated the CSS style to better match what we're doing elsewhere, including:

• Explicit white instead of #fff
• No spaces in GtkCSS functions

I left the white active state for the active display for now; it looks no worse than before, but I feel like there is probably something better to do. Targeting .colored.background for the on-screen identifier also feels a bit… weird. Input welcome.

This branch

Light	Dark

master

Light	Dark

Intended to fix #259

[danirabbit]

So I was thinking about this the other day and I wonder if this is another case where we could be using @selected_bg_color

[cassidyjames]

@danrabbit u so smart

[jeremypw]

@cassidyjames This PR seems to have stalled but has support - any chance of finishing it?

[cassidyjames]

@jeremypw I haven't actually thought through how to change it to use the stylesheet variables @danrabbit mentioned; I think it might require more of a refactor than I was attempting originally. I'd be fine either merging this as-is for now since it's at least an improvement, or pursuing the other way when someone has the time to do so.

[jeremypw]

@danrabbit I do not know enough about stylesheets to help with this - can you? Or is it worth merging as is and improving later?