I would like to experiment with some settings (css is hard ;)) but it looks likes that only way to see changes is to restart server (rstudio-server restart). It is very inconvenient to test changes in that way on the production server.
I totally feel your pain!! Restarting things to iterate on css, especially, is super rough!
There may be a better workflow, but for poking at css, I usually just open up the Browser Devtools (in Firefox, Chrome, Brave, etc.) and go to "Elements." It takes some getting used to, but you can typically monkey with css directly in the browser and get immediate feedback.
Article on how to open in Brave/Chrome, for instance:
This usually gives me enough of a start to go and build the actual css on disk, and then move to the slower iteration cycles. Two other tips:
SelectorGadget is a browser plugin I have used in the past to look at what CSS selectors are for a given element. It is most useful for web-scraping, but can also be useful for this purpose
You can also get similar functionality by clicking on an "element" in the browser devtools and selecting "copy selector"
As you wrote this is a partial solution, but solve some problems.
I was also thinking about an alternative: setting up a virtual machine (with the free RStudio Workbench version) and testing the login page there. Then I can restart it without worrying about interrupting the someones work.