[XKit Preferences] Dark Mode Update 2

[motackt]

Redid #1848 with the current master.

Original Description:

Put in some logic (using checkboxes) for the XKit Control Panel to:

• Match the current Tumblr colour palette. True Blue, Cement, and Canary all tell XKit to use the default light theme, while Dark Mode, Low-Contrast Classic, and Cybernetic all tell XKit to use this new dark mode.
  • So if the Tumblr colour palette changes, then the XKit Control Panel will update next time the user opens the Control Panel.
• Always use dark mode regardless of current Tumblr colour palette.

Updated some class styles with items that weren't previously defined and so defaulted to black (not very clear against a dark background), and also added more variables because white box-shadows are not really a great design choice in dark themes.

As a bonus, changed the default and kernel extension icons to SVGs for crispness and space!

[nightpool]

Just a few quick notes—i don't have a lot of context on the current state of this code, i might not be the best person to review the rest of it

[nightpool]

we're changing a lot of -shadow variable usage to -overlay in this PR. going forward, when would someone use -shadow vs using -overlay? is -shadow superfluous now?

[motackt]

I replaced all --shadow variables that do not describe a box-shadow or text-shadow to be an --overlay instead. I did this for two reasons:

1. It's more descriptive this way. --overlay denotes something that will colour the entire element (ie hovering over a tab), whereas --shadow is now only used for shadows (ie text-shadows and box-shadows).
2. Dark Mode inverts the shadow colours (ie --shadow goes from black to white, and --white-shadow goes from white to black). But not all shadows should be inverted, such as --shadow (which goes from black to white) because it creates white box-shadows which look really odd because they're not as intuitive as black shadows). But some overlays are still useful, so I split them up into --shadow and --overlay so that I can invert --overlay while turning off --shadow.

If you don't do this (ie no --overlay variables and invert every shadow), then you get this result:

Weird white box-shadows.