I have found that the less files are not compiling in my custom theme. at all. originally I discovered the problem when I made changes to my _extend. less file and no changes occurred, but now I have found that none of the .less files in my app/...web/source/CSS folder are compiling after the first loading of my website.
After downloading a fresh backup, I immediately made some changes to the CSS before opening the website online. this worked, but any consequent changes had no impact. I reversed the changes I had made and re-loaded the page by opening a new window. This reverted my original change, but any time after this no change is evident.
I am using xampp, with Magento 2.3.0 installed.
I have tried every fix found on this website and regular stack overflow, including clearing the cache, var folders and more using both commands and straight-up deleting the folders.