Use a more visible color for selected thumbnails (bug 1690428)

[calixteman]

In HCM, the thumbnail ring wasn't visible at all.

[Snuffleupagus]

In HCM, the thumbnail ring wasn't visible at all.

Did you actually mean to limit this to that mode? As-is this also affects the "regular" light/dark theme where it looks really out of place (and dare I say quite bad).
Furthermore, this color is also used in the outline and there this really doesn't work at all (it's especially bad in the light theme).

[calixteman]

In Firefox it seems that it's the color we use to outline:
https://searchfox.org/mozilla-central/source/toolkit/themes/shared/global-shared.css#19

And I agree with you but in the meantime I agree with Mats (who filed the bug) either.
I'm using myself a dark mode and there is this blue line which doesn't really hurt me:

I thought that it's better to use the same kind of color everywhere because the user is used to them.

We can limit the change to HCM.