Whenever I link to a stylesheets within the CSS folder of my theme, the file is not found.
Actually, as long as you have your styles in the Global folder of the theme, as in the attached image, you don't need to reference them again with a css widget. they will be loaded authomatically. If you need to load external css files (which will not be part of the theme) you can use the widget, but I will recommend you to place those styles in a folder with a different name and outside the theme, because you may experience some permissions problems.
Thank you Jen. The stylesheets I'm trying to load are not meant to be part of a theme, so I'll try to add a different "Styles" folder on the same level as "Global".
I was confused because the folder structure that Sitefinity suggests puts a folder called "CSS" on the same level as "Global" and that's what I'm trying to use. I'll see if renaming it helps at all.
I solved my problem. The issue was that the Sitefinity widget was not entering the right path. This could be just our installation and configuration.
When I selected a CSS file, the path would be something like "dev.domain.com/.../styles.css".
Sitefinity wouldn't find that file. So I removed everything from the beginning of the URL through App_Data, leaving "/Sitefinity/...". This works.
I'm not sure why Sitefinity is applying incorrect URLs. Again, it may be our installation, but we setup our structure to match Sitefinity's suggestion: www.sitefinity.com/.../website-templates-file-structure.
I'll look into configuration settings on this on my own, but can anyone point me to a spot where this path adjustment could be made globally?
The structure you refer to is correct, and for themes it is the correct folder path to input (www.sitefinity.com/.../registering-a-theme).
A CSS Widget however is just like a regular link tag, as in it has no access to the /App_Data folder.
To accomodate for that, Sitefinity basically 'rewrites' the path and removes the /App_Data part of the url.
So both dev.domain.com/.../.... and /Sitefinity/... would work, it's just a .NET restriction that you can't reference the App_Data folder.
Thanks for the feedback. This is very helpful.