beautiful-jekyll-next: add theme switcher

### Is there an existing issue for this item?

- [x] I have searched the existing issues

### Repositories

LizardByte/beautiful-jekyll-next

### Languages/Skills/Technologies

JavaScript, HTML, CSS

### Description

Add a theme switcher using bootstrap 5.3+.

### Estimated Effort

effort:Small

### Priority

priority:Medium

### Target Milestone

1-3 months

### Dependencies

_No response_