Add light mode colours

[seanmturley]

Description

Add light mode colours. This should attempt to use the existing dark mode colours as much as possible - ideally just assigning the same colours to different roles, though this is likely going to run into issues.

Issues to resolve

• [x] The colours for Filters don't work (dark mode) - probably just requires a background colour
• [x] The colours for Filters don't work (light mode)
• [x] The "Loading..." overlay is almost unreadable - probably requires white text override

[seanmturley]

Added a first pass at the light mode colours. It's working pretty well, with just swapping the existing palette around to different roles, and adding a single extra shade.

There are a few issues to address, which I'll add to the main task description.

[seanmturley]

There was a --black variable being used for the background of main that was only visible under the filters and behind the full page view of a post. Changing this to the slightly lighter --back colour that already existed fixes the colour issue for dark mode Filters, and also generally produces a cleaner look.

This also improved the colour issue for light mode Filters, but it's still a bit lacking in contrast, so further tinkering is required.

[seanmturley]

The problem with the Filters colours in light mode was a lack of contrast for certain elements. After some failed experiments with darkening the --mid colour for better contrast on the white background, the best solution ended up being adding a few CSS colour overrides to replace --mid with the slightly dark --mid-weak when [data-light-theme] is enabled.

[seanmturley]

Added a [data-light-theme] override for the "Loading..." text so it's visible (previous contrast was almost zero).