Heather Solomon has earned her place as the definitive expert on SharePoint branding, if for no other reason, due to the excellent technical articles on her website. One example is "CSS Options with Master Pages" which is exceedingly helpful. Still, as I'm new to SP development, I've found I had to read the pivotal paragraph (which starts "a general breakdown") several times. I needed to dumb it down to get clarity:
* If your master page links to a custom CSS file,
Custom BEFORE Core
* Master pages deployed via a Feature,
Custom BEFORE Core
* If your master page links to a CSS file, and CORE.CSS is unghosted,
Custom AFTER Core
* If you specify a custom CSS file in the Master Page settings (Site Actions >Site Settings >Master Page),
Custom AFTER Core
* Any styles listed or linked to in the PlaceHolderAdditionalPageHead ASP content placeholder,
Custom AFTER Core
* Styles listed explicitely with style tags in the master page file,
Custom AFTER Core
Having got that straight in my head. My next step is to learn how to apply custom Themes.
It is obvious that if you specify a custom CSS file in the Master Page settings (Site Actions >Site Settings >Master Page) and then apply a Theme, the Custom styles come BEFORE Theme styles. Mmmmmm…
seanharrison