consider prefetching other theme stylesheets

[enketo-issue-mover]

From @MartijnR on May 24, 2018 22:49

• [ ] check if considered a violation if those stylesheets are never used

Copied from original issue: kobotoolbox/enketo-express#1036

[enketo-issue-mover]

From @MartijnR on September 10, 2018 23:8

Solving the theme-swapping delay would probably be the most non-enketo-core related loading performance opportunity.