I have recently upgraded to M2.3.3 when I got to "Content > Design > Themes" and try and save my custom theme I get the following error. It's also ignoring the changes I've made in Magento_Theme/layout/default.xml not sure if this is related.
Path "/theme/origin" cannot be used with directory "/public_html/pub/media/"
Check your theme table from database. If your theme is of type 1 or 2, change it to 0 and it will work after that. Magento is using now different types of themes which are 0 = physical, 1 = virtual, 2 = staging.