Dark theme buttons

[srprkr]

Required for #2193

Changes Summary

• Changes buttons/input box shadows for better support in dark mode.
• Removes unused .dark class selectors in css files and nests corresponding rules in a prefers-color-scheme:dark media query to activate those intended styles
• Dims placeholder text of custom donation box when selected.

This pull request is ready for review.

[srprkr]

I removed the conflicts. I just opted for what came from master since that was code I didn't touch in my changes. Let me know if that was wrong.

[srprkr]

Sorry for the messiness. I have the stylelint issues fixed now.

[RMcNeely]

Hi Sean, we're in the process of closing stale branches if you're still interested feel free to reopen and resolve the conflicts