[plugin-architecture] Use variables on breakpoints

[tiagoalvesdulce]

Investigate ways to use variables on our breakpoints. We can't use CSS variables.

Alternatives:

• Use custom media queries - depends on PostCSS
• Use postcss breakpoints - depends on PostCSS

[tiagoalvesdulce]

• Use CSS Environment Variables. Can I use? page showing great support from modern browsers

[tiagoalvesdulce]

@victorgcramos what's your opinion?

[victorgcramos]

Sounds good! I'd go for CSS Env vars. IMO it's a cleaner solution

[tiagoalvesdulce]

Closed by https://github.com/decred/politeiagui/pull/2849