Normally, I refrain from joining forums because of the toxicity that most forums have, but your message made me want to congratulate you on this. Now I can customize Krita to my heart’s content.
With themes, it depends on your OS that you did not name, also your version of Krita is missing, as well as from where you downloaded it, because this can make a difference in how Krita is installed and sometimes needs different approaches to solve an issue.
This is the usual way, but it may not work with your unknown system:
Not bundled resources are installed into Krita via ‘‘Settings’’ >> ‘‘Manage Resources…’’ where you have to click on the button Import Resources and then browse to and select the resource in question, click on okay, or however a selection is confirmed with your file manager, and the resource is installed. In seldom cases, you may need to restart Krita afterward, but usually you can use the resource right away.