fix(css): fix issues visual issues with dark theme

[SethFalco]

• Make the error message UI use the secondary background color defined by our theme.
  • This fixes the theme on dark mode.
  • This changes the color in light mode to use the same color as the text areas and file upload UI.
  • UI looks more consistent with other components.

My previous PR introduced the other issues. Sorry, I should QA better. ^-^'

• Fixed a bug where the textareas had black text on dark mode. It's now white on dark mode like before.
• Fixed a bug where the language selection didn't apply the correct background color on dark theme.

Screenshots

Before (libretranslate.com)

After

Reference:

• chore: clean up theming #454

[pierotofy]

Thanks for the fix! I should have tested more thoroughly as well ^_^