I've built an unstable theme?

I am building a series of WP sites. All 5 should look very similar, as they are branches of the same small business. The owner chose a free template for me to work from (it was built by WPExplorer, so I had a certain amount of confidence in it). I have build child themes of the original for each of the sites.

But since the WP update, the main site has become extremely unstable. For example, I tried to change a font size this afternoon (using only JetPack’s CSS editor), when I refreshed the page to view the change, all of the style was gone. The style came back for about 20 minutes and then was gone again, and still is.

Is there an easy way to find out what the problem is–why this theme is now so sensitive to the smallest change? Please tell me it’s fixable, I don’t yet have the ability to build something like this from scratch.

The site that’s giving me trouble is https://www.uniquewebcopy.com. An example of another site I did on the same template is https://www.blogpostsforlawyers.com.