Add button for guests to toggle forum theme

[dsevillamartin]

Fixes #35

Changes proposed in this pull request:

• Add button to HeaderSecondary for guests only

Reviewers should focus on:

• Button shows current theme because there are 3 options, showing the next one is more confusing

Screenshot

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).