How to Reload a Custom Drupal Theme (Without the Command Line)
The steps that so far work reliably for me with Drupal 8.9.1 to load a custom theme again without command-line access are:

Install and enable the Devel module. (You will only need to do this once.)
Put your site into maintenance mode.
Turn on Devel -&gt; Settings -&gt; Rebuild the theme registry on every page load.
Clear the caches.
Load the new copy of the theme from Appearance -&gt; Install new theme -&gt; Upload a module or theme archive to install.
Clear the caches.
Reload one of your pages to verify your theme changes.

The second &quot;Clear the caches&quot; (6) might be redundant.
So far, this has worked like a champ once I figured out all of the necessary steps.