Development & caching are always biting each other a bit. This is caused by the fact that IIS considers your .css as static content in a virtual (non-existing) folder.
If you go into web.config and comment out this:
<clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="31.00:00:00" />
and recycle the application after that, things will probably go smoother during development.
If you rather not change the web.config and clearing your browser cache still doesn't work you can try the following 4 hacks:
1. Publish the page, clear browser cache, edit the page instead of merely open and going back.
2. Rename the .css file, refresh the page (can't find .css) rename the .css file back, refresh the page.
3. Open web.config add a space, close it, refresh your page.
4. Recycle app-pool.