VPN-6213 (part 1): Remove explicit colors

[mcleinman]

Description

VPN-6213 is to refactor main theme to separate layout/spacing from the colors, and while we're at it remove explicit colors from the code.

This is going to be done as 3 PRs, stacked on top of one another:

1. Change explicit colors to variable colors (this PR)
2. Change use of variable colors in the code to named variables, and set them to colors (to make them easy to swap out for other themes) - so instead of a border being grey20 it's a new variable borderColor, and borderColor is set to grey20.
3. Pull colors out from the rest of theming, so it's easy to swap in new color schemes as desired

For this PR - some of these color choices are pretty straightforward (looking at you, #FFFFFF and #000000), but some were slight judgement calls. I tried to match QT explicit colors (in MZAlert and another place or two) the best I could to our color scheme. The oddest looking one is on MZSubscriptionOption, the glow color. I spent a while testing different options, and think grey30 is best. The image on the left is how it currently looks in the app, the one on the right is grey30:

Additionally, I only focused on client colors for this PR. Areas I found that still have some explicit color:

• images/animations/etc.
• app icons, notification icons, mobile launcher screens, etc
• documentation including mozillapvn.dev
• inspector
• translation report
• linux flatpak webpage
• macOS setup screen
• WASM's chrome

I'm going to ensure we have tickets filed for these first two (update: this has been filed as VPN-6681), but the final 6 are items that I don't think we need to make generalizable (so they may never get a dark mode).

Reference

VPN-6213

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed

[oskirby]

This one is a bit odd because there's an alpha component here, which I think translates to grey10 but very faint and the alpha blending is lost by calling it "grey10"

I don't actually know if the alpha here matters at all or even makes a visible difference.

[oskirby]

Looks pretty straightforward.

[mcleinman]

There were a couple places I pulled the alpha channels. With one color scheme we're getting the same results, and I think moving towards multiple themes it is probably best to remove alpha channels as part of the color strings.