Support setting masthead height in a theme

[ksuderman]

The AnVIL instance of Galaxy makes some UI changes that cannot be set in a theme, in particular the --masthead-height and --masthead-logo-height. This PR adds support for those two CSS variables as well as a sample AnVIL theme that makes use of them.

How to test the changes?

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[guerler]

Thanks for working on this. Overall this looks fine to me, but I would prefer masthead-logo-height to not be a theme variable, but rather derived as ratio of the masthead head height as you suggest above.

[ksuderman]

Right now the height is optional and is only included in the sample theme until I can figure out how to get it to be recalculated. I was hoping @ElectronicBlueberry's suggestion would fix that, but I'm still seeing the value for --masthead-logo-height to be 2.5 * 0.6 and not 3.0 * 0.6 as it should be if it was using the masthead height as defined in the theme. Laila, do you have any suggestions?

[ksuderman]

What I am trying to achieve is to get as close as possible to what if defined in AnVIL's Galaxy fork: https://github.com/galaxyproject/galaxy/compare/release_24.1...anvilproject:galaxy:release_24.1?expand=1#diff-1bd3a4e46a82fb1d44b3eb90e15598c39de297524b5cde9646f9189bccae8198