When accessing Theme Builder from a Modern SharePoint subsite on Office 365, it is in the loading state indefinitely. It does not proceed to show options to choose between modern and classic and customize your site.
You have not upgraded to ShortPoint SPFx version 126.96.36.199 through the App Catalog Upgrade (not through One-Click Upgrade).
First, Verify ShortPoint SPFx Version
Go to Site Contents of the subsite, click on show actions ellipsis beside the ShortPoint SPFx app, and choose Details in the drop-down.
If you see version 188.8.131.52 or lower, you need to perform the steps described below to resolve the issue. If you have version 184.108.40.206, please contact us at firstname.lastname@example.org.
Note: In the present case, please do not check the ShortPoint SPFx version by going to the Dashboard, as it will show the One-Click upgrade version, but will not show the latest App Catalog upgrade version.
Next, Perform A Full App Catalog Upgrade of ShortPoint
You can follow the steps shown in these support articles to perform a full app catalog upgrade with version 220.127.116.11.
Once you have upgraded to ShortPoint SPFx version 18.104.22.168, Theme Builder will be accessible to edit your Modern subsite.