A less vibrant version for the UI theme

[tinacious]

The newest (April 2020) version of the theme releases 2 new vibrant UI themes.

A more subtle theme would be great to have too, e.g. greys, etc., provided that they use the same greys in the existing palette.

Suggestions for contributing

Please do not modify an existing theme.

If you'd like to contribute a new theme, please run the colour scheme by me first with your ideas before getting to work. A requirement is that you provide both light and dark versions.

Please also do incremental commits, i.e.:

1. Copy an existing theme and commit that, without changes
2. Make incremental changes in commits so I can see how you've changed it from the original

The screenshot in the provided PR appear to be on-brand enough and along the lines of what I'd be looking for: #3