www/rubygem-bullet_train-themes: Add rubygem-bullet_train-themes 1.7.2

Bullet Train has a theme subsystem designed to allow you the flexibility to
either extend or completely replace the stock "Light" UI theme. To reduce
duplication of code across themes, Bullet Train implements the following three
packages:

• bullet_train-themes
• bullet_train-themes-tailwind_css
• bullet_train-themes-light