is doctype really necessary? and when do you know which doctype to use?
If you're interested in writing valid HTML, then yes, it's necessary. Use whichever is appropriate for the version of HTML you're writing. XHTML is the current recommendation, and has been since 2000, but if you don't know, you're probably using some form of HTML 4.1. I'd personally recommend learning more about HTML and writing valid code.
also, how do you change the scroll bar color in FireFox?
You don't, and this is a good thing. Keep your design to the page itself, don't mess with your viewer's UI elements (scrollbars, status bar, mouse pointers, etc).
also, Firefox is not registering any of the CSS that is embeded into the pages.
The first thing to do is make sure your HTML and CSS are both valid, then troubleshoot from there.
If you want useful replies, ask smart questions.