Race condition leads to dark mode not being applied

[rugk]

Bug description

@lionirdeadman reported a bug with a specific website that triggered

Steps to reproduce

Dark mode only used in pure CSS. Same CSS on each site.

But on sites with much HTML content,

I currently guess this is a timing issue, because the add-on tries to get the CSS from the website too fast, so it has not already loaded (because it's still parsing HTML).

Actual behavior

Sometimes fails to apply dark mode.

Expected behavior

Always work.

System

Browser and version: Firefox 68

Possible solution

trigger later?