feat: Normalize filenames, simplify themes.json

[AdisonCavani]

Description

To avoid over-typing, simplify the filename convention.
Now all assets/backgrounds and preview images have the same name.

Normalization - names are slugified.
Example tool: slugify.online.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project (correct file and folder structure)
• The background image for theme follows naming convention: theme_name.png
• I updated themes.json file (JSON database)