If your custom theme layout xml and css not working then check your Database. There is a table called “theme” that stores a reference to all the themes that have ever been installed in your store. If the type of the theme is set to 1, that means Magento sees the theme are virtual and will no longer load your xml files. If that is the case, just change it to 0. I don’t know the why of this issue, but it has tripped me up a few time.
And after that clear “cache” folder inside “var” folder and also delete “files” inside “pub/static” folder and then run static content deploy command.
Command: php bin/magento setup:static-content:deploy