Canvases don't redraw when system theme changes

[mstange]

Steps to reproduce:

1. Go to https://profiler.firefox.com/ and make sure your light/dark mode setting is set to "Follow system theme preference".
2. View a profile, for example https://share.firefox.dev/4a0LssO
3. Change your system theme.

Expected results:
The flame graph and activity graphs should update with the new colours.

Actual results:
Canvases keep the old colours until they redraw for some other reason.

┆Issue is synchronized with this Jira Task

[canova]

I noticed that too. We should force them to redraw. Thanks for filing a bug!