Transparent background on some elements when using `linear-gradient`

Summary

When customizing the theme with custom CSS and using linear-gradient some elements' / sections' (Inbox, profile preview, message actions, attach menu, etc.) background becomes transparent.

Steps to reproduce

1. Go to Settings > Look and Feel >Theme> Custom Theme Tokens > Custom CSS Overrides
2. In the Custom CSS Overrides write :root { --background-primary: linear-gradient(to bottom, rgb(0, 85, 150), rgb(15, 200, 170)); }
3. Check out the elements' / sections' background, in particular: Profile previews, Inbox, Message Actions, Attach to message menu, The section of the emoji menu that shows the emoji's name. (These are all the sections I found which backgrounds are broken)

• Note: I got the fluxer-bin package from the AUR.

Environment (optional)

AUR Arch-Linux

Logs or screenshots (optional)

How it is intended to look (Opaque background): image How it looks when using color gradients (Transparent): image