Clicking on "change the look" of a modern site opens the panel as normal, but the side panel only has the current theme, and no others. Where there used to be a list of about 12 different color themes, there is one: current theme. This is the case in two different tenants. Did I miss an announcement? Any suggestions on why only one theme is being displayed?
Looks like this issue started appearing since yesterday.
As a workaround, for now, you can select your themes from the modern list/library pages and Site contents page. The themes are still there, just not visible at the moment because of a bug.
Good news is that MS is aware of the issue and will be fixing it.
You can monitor this issue on github issues list.
Also, to get a list of themes, you can use
Get-PnPTenantTheme and it will list the available themes in your tenant.
Reference - Modern Themes Broken