Add remaining dark mode colors (fixes #394, #395)

[Keltena]

This pull request:

• Adds a dark mode palette for the .alert-danger and .alert-success infobox classes (based on existing dark mode colors for alert-info), and the .btn-danger and .btn-success button classes (based on existing dark mode colors for .btn-primary)
• Changes the dark-red warning label on form fields with errors to light red in dark mode
• Fixes a few borders I missed last time, as well as a place where buttons were falling back to light-mode colors

This fixes #395, and should fully fix #394 if I haven't missed anything.

[coveralls]

coverage: 65.299%. remained the same
when pulling 588419f on Keltena:css-fixes
into b60f63b on iftechfoundation:main.