CSS for a custom theme should be selected/loaded automatically.

[tdonohue]

Describe the bug
Discovered during the review of #1063 (comment)

When creating a custom theme (or even the "dspace" base theme), it is currently recommended to modify the src/index.html to reference your theme's CSS directly. See https://github.com/DSpace/dspace-angular/blob/main/src/index.html#L10

Expected behavior
Ideally, this CSS should be selected automatically and/or written into the file by webpack, for example https://medium.com/dailyjs/inserting-variables-into-html-and-javascript-with-webpack-80f33625edc6

This would make it easier to enable a custom theme just by modifying configuration (and no longer needing to touch the main index.html).

Tentatively scheduling for 7.0 as this seems like something we may want to clean up prior to final release.

[artlowel]

I created PR #1212 to fix this issue and #1134. It took 2 hours