Branding Palette

[rgaudin]

In addition to the Branding horizontal logo and square logo, and in the light of offspot/dashboard#13 and offspot/dashboard#12, we'd want to offer additional branding-integration.

Allowing users to select and/or create a custom color palette seems pertinent:

• It's easy to pick ; requires no technical knowledge
• It can strongly reinforce branding by using matching colors
• we could (at some point) propose an auto palette from the logo's colors
• It's easy to store, carry and reuse: it's versatile.

This requires a couple of upstream features:

• Defining the Kiwix default color palette from the new UI: how many colors?
• OCI images (dashboard, captive portal) to include an arbitrary (in branding/) CSS file
• OCI images to use CSS variables that would be overridden by this file