Removed default anchor CSS filtering in dark mode

Removed default anchor CSS filtering in dark mode

Due to causing content images to be rendered in unexpected ways.

- Also removed CSS filters from other image usage.
- Tweaked header CSS filtering to not be so aggressive.
- Forced WYSIWYG editor to be on its own layer since that would allow
massive larger performance increases in Safari, especially when using
dark mode.

Closes #2045.
Closes #2154.