Thank you for reaching out. Have you verified the themes are compatible with the Android version on your device?
Are you using dark mode on your device?
Have you had a software update recently to your device?
Do you have dark mode on? For some reason, certain apps don't theme while dark mode is activated.
Yes!! It has been an ongoing problem for me for over a month, and on and off again for probably 6 months or more. Try switching to default, then delete the theme u want to use from 'my stuff' , clear the cache and data from Themes store, restart your phone and then redownload the theme and apply. If that doesn't work, try clearing cache/partition from your device. Last thing to try if those things don't work is clearing the cache and data from Google play services (settings>apps) and also Android web view. The last step seems to be what helped resolve the issue for me, really think there is a glitch and/or compatibility issue with them and latest versions of Android.
I have several of the themes u mentioned 😁 U mentioned Sebastian, another excellent theme developer who currently has several of his themes free or discounted is Alex Pie.
The themes you have installed might not be fully compatible with your OS version. Try these themes and you should have a complete theming solution: