yes there is.
https://www.travzilladesigns.com/version3/wp-content/themes/jobforacowboy/style.css
This stylesheet, assuming it’s the one you are trying to use needs to have some info at the top of it, similar to what the defualt theme has:
/*
Theme Name: WordPress Default
Theme URI: https://www.ads-software.com/
Description: The default WordPress theme based on the famous <a href="https://binarybonsai.com/kubrick/">Kubrick</a>.
Version: 1.5
Author: Michael Heilemann
Author URI: https://binarybonsai.com/
*/`
That information is how wp knows what the theme name is, etc.. within the presentation tab. Without some sort of info there, my experience is that wp resets the default theme as soon as you click on the presentation tab.
Thats ONE cause, there might be others, but thats a good place to start.
***Do not copy and paste my example, and then change the content unless you pay attn to removing the backticks that wouldnt go away from my own copy and paste.