All themes are compatible with iOS 13.3 and iOS 14, I have tested it myself also to apply theme download the Snowboard app which is available in Cydia.

Top 10 Beautiful Cydia Themes For iOS 14 List:

Muze 4
Veexillum
Pulsar
Classic iOS
Mint 3
Oyster
Ace El Cap
Lotus
Amury
Jelly 3

Can you install themes on iPhone?
Once your iPhone device has restarted, all you have to do is browse installed themes. To do this, you need to open the section Select Themes and check the boxes of the themes you want to customize your iPhone. You can at any time download more themes when opening Cydia and by going to the menu sections.

How do I put themes on my jailbroken iPhone?
How To Apply Themes with Snowboard?
Open Cydia.
Search for the theme you want and install it using Cydia.
Go to the Snowboard in Cydia settings.
Tap the button that says the select theme.
Tap and enable the theme of your choosing and then click on Apply.
Your selected theme will be applied to your home…