Settings: Change background buttons

[BurkusCat]

There are currently 4 historical backgrounds in the repository:

• background-image: url("../img/site/steel-wave.jpg");
• background-image: url("../img/site/void-edge.jpg");
• background-image: url("../img/site/gray-triangles.jpg");
• background-image: url("../img/site/black-mist.jpg");

It would be good to have arrows to cycle between the available backgrounds in the settings menu. It should be remembered for future visits to the site.

Example of how the UI could look:

Background:
____________________________
|   ◀    Steel Wave    ▶   |
-----------------------------