I can’t be sure, but I think my site predates the current theme system, and used to have a single theme, and it was Sam’s Simple theme at the time, and then things got standardized and the short of it is I don’t have a Light or Dark theme, so are those in a repo somewhere I can grab and install?
You should be able to create a new theme (via the install button on admin > customize > themes) call it “Light” and select the light color scheme from the theme’s Color Palette dropdown.
The light/dark themes don’t have any custom styles, they’re just blank themes with a color palette selected.
The Light color palette is always available as the default option, but if you need to recreate the dark palette the colors are:
Alternatively, you can re-run the color step in the wizard to create the themes (discourse.examplesite.com/wizard/steps/colors)… just note that this also changes the default theme (which you can change back via the admin panel of course, but depending on how active your site is you might not want to suddenly change the default for everyone).
For anyone else that wants to get the newer themes, here is another solution. If you have another instance that has the new themes, you can export them from the admin ui. Just go to the theme and click export. Then upload them to your old forum using install -> from your device. Just did it now and it worked well.