Hi,
Weird behavior here.
Here’s how my forum usually looks :
I have a custom light palette named Monocycle Jour, and a dark one named Monocycle Nuit.
I installed the Dracula theme via the Install button from the admin interface, and my custom palettes were gone. I didn’t even activate the Dracula theme.
My forum now uses the default Light theme:
And the custom palettes don’t appear in the color palettes page, or my theme palette selection anymore:
There’s nothing in the logs and staff action logs as well.
Opening the palette using their ID like https://forum.monocycle.info/admin/config/colors/3 returns no palette.
Discourse version is 3.6.0.beta2-latest(398e04181f).
How did this happen? 
Is there a way to get my colors back? By chance I still had my light theme open in another tab so I could get back the light colors through the browser’s dev tools, but not for the dark scheme.