Convert gallery scss to css

[body-clock]

✅ Tested working on Blacklight 9 (main branch)

I think the only consideration here that potientially needs a bit of discussion is changing this:

  .index_title {
    @extend .h6;
  }

to

  .index_title {
    font-size: 1.25rem;
  }

Unlike the .caption class, the .index_title class is used in Blacklight core - this would involve a template override. I think the potential maintenance overhead of an override is significantly more than evaluating whether a font-size of 1.25rem is close enough to the Bootstrap .h6 class. I tested in browser and I think they look similar enough. I'm certainly open to more suggestions, though.

[cgalarza]

I didn't think we wanted to merge this yet because it uses CSS variables which aren't supported in Bootstrap 4. We wanted to cut a version of blacklight-gallery v4 before merging and moving onto v5.