Is your feature request related to a problem? Please describe.
Maintaining a theme for Janeway is a lot of work, because many new features are facilitated by changes in the theme files. Custom themes usually only change a few templates, sometimes only one.

Describe the solution you'd like
Janeway themes are already based on the "clean" theme, it should be fairly simple to allow us to specify which base theme to use. This is, according to Andy, just a few lines of code.

Describe alternatives you've considered
Keep maintaining a full set of theme files, OR keep sending feature requests until we no longer need the customized theme.

Additional context
n/a