CSS

Add an external stylesheet to magento using local.xml 

Create/edit the following:

app/design/frontend/PATH/TO/YOURTHEME/layout/local.xml

Make it look like so--should be self explanatory...

I found an interesting discovery with Notepad++ where PHP, HTML, CSS, etc language syntax highlighting didn't appear in the language menu. Not sure if this is the result of the way an older version set the settings--whereas, even if you upgraded to latest version (without uninstalling first), the upgrades wouldn't overwrite the original settings file. I think this was disabled at the time of install when it asks if you want it to be your default for HTML files; where I normally UNselect this, since I use a PHP IDE to handle PHP, HTML, CSS, etc files.

Anyway... The fix: