Theme: get closer to Open Food Facts look & feel

### Story

The current UI feels too much bootstrap-y. 
We should use more of the OFF colors.

### Useful links

- https://world.openfoodfacts.org
- https://hunger.openfoodfacts.org

### Todo

- [x] change the primary colors (both light & dark themes)
- [ ] change the font
  - `"Public Sans",Helvetica,Roboto,Arial,sans-serif`

