--tab-line-color should be "highlight" by default on Linux (Photon-style)

[Alex-gnzl]

Description:
Currently the tab highlight is hardcoded, making it blue on the system theme.

Expected behavior:
Should follow active GTK theme.

Screenshots

Environment:

• Distribution
  • Original Lepton
  • Lepton's photon style
• Firefox Version: 89.0.2
• OS:
  • Linux
  • Windows
  • Mac
  • Others:
• Theme:
  • Light
  • Dark
  • Alpenglow(System: Light)
  • Alpenglow(System: Dark)
  • System Default
    • GTK Theme Name(Only Linux User): Breeze GTK

Additional info:
This does not matter for custom themes, only System Default.
https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#system_colors
(i have tested this with Windows 10, and alas it doesn't work there.)

[black7375]

I was thinking of doing more GTK compatibility work after the dark mode.
For example, URL bar border, etc. should also be GTK highlight color.

Nice report.

[Alex-gnzl]

While I'm at it, might as well specify this:

• there isn't much jank regarding color otherwise, most things just work
• urlbar seems to have a highlight border when i open a new tab, but it's darker for some reason (i'm not sure if it's focused or not? it's odd - focused but not unfolded; might be simply broken on my firefox)
  
• Highlight and HighlightText seem to be the only useful ones out of the bunch (see comparison screenshots between two GTK themes, Breeze (w/ custom colorscheme) and Adwaita Dark)
  
  
• Changing to a theme that isn't "System Default" does not affect "Highlight", but thankfully --tab-line-color gets overridden to the theme color, so it just works.
• For me, using Highlight on Windows 10 just defaulted to some blue color, I think? Either way, it's not usable for Windows. Sadly Firefox doesn't support it.

Thank you for this project!

[black7375]

Now, GTK Supported at develope verstion!!