There is something very odd about adding additional stylesheets via resources and injecting them … I think it’s broken.
 Using Foundry framework (but could be anything this is not related to Foundry as far as I can see) create a simple page, add simple content, Eg, a paragraph stack.
 Create a stylesheet with a single class like;
 Add the stylesheet as a resource then link to it using the macro in the global or page head container.
 Go back to the paragraph stack and add class ‘customCol’.
 Preview, export whatever… this MAY work.
Now, open resources and select the custom CSS file. Choose edit with xxxx (E.g. Sublime ). Change the class style directives to use Eg blue text. Whatever, it does not matter. Save the stylesheet.
Save and preview or export then preview:
That the edited and saved class style rules will kick in and the text will be blue (or whatever you configured in the .css).
The previewed page does not reflect the changes and the text will still be red, or whatever it was before. Your updates styles are not honoured even though you have saved the file, closed out of resources and hit ‘preview’.
Want the kicker?..
If you then change any other element on the page, using stacks, for example modify the text in an unrelated header, or anything minor like that… the CSS change you made in your stylesheet is then picked up and the styles are applied. What you changed does not need to have the custom class you added in the css file. Any change, as far as I can tell, will do.
Clearly the rebuild of page styles including possible external resource based stylesheets is not being triggered until there is a conventional page ‘edit’. Altering your resources based stylesheet isn’t calling that rebuild/render. Why?
RMS… what IS going on…? This CANNOT be expected/reasonable behaviour… you surely need to wire up page update routines (especially for *.js or *.css) when an edit has been made to resources NOT rely on subsequent edit to page. As it is, it’s broken, in my opinion because I can’t just edit css in a linked stylesheet and expect it to work without some arbitrary edit on the page.