The Mantra theme for Drupal is simple and minimal.

Utility classes allow customizing components via the UI.

Inspiration

https://getbootstrap.com/
An excellent front-end framework.

https://designsystem.digital.gov/
A practical framework for government-wide theming.

https://tachyons.io/
A minimalist design library.

https://www.radix-ui.com/
Accessible base components.

https://ui.shadcn.com/
Popular component library.

https://every-layout.dev
Excellent layout guide.

https://100r.co
Cool b&w site.