Fix Alpha color themes

[mmmmmob]

Description

• Fix some color options which previously using placeholder that caused some error on plugin like Firefox Gnome theme (some parts shown as transparent which is hard to read)
• Fix Custom CSS to make symbols on windows manager (close, minimize button) visible (only on Dark, Black and Tritanopia -- the Mac version I think the creator intended to use color as symbolic)

Screenshots (optional)

Checklist

Before submitting the PR, I checked:

• [ ] I've only modified one preset
• [x] I've checked the format of each json files I modified
• [x] (optional) I've added screenshots
• [x] The preset name is following preset naming guidelines.

Thanks for your submission we will review and merge it as quick as possible.