HTML5 CSS Beginner

Hi Guys, i just started Treehouse and i love it, it is really the way my brain learns best.

Im at the point in the course that we implement Pictures (stage 3, Creating HTML Content). I went ahead of my self and thought im gonna do something that suits my purpose, so since im traveling in a week trough the alps to zürich i thought im gonna modify the code so instead of Portfolio, About and Contact im gonna use To do, Route and Checklist. So i created a background for each of those three pages, and in total six buttons, for each page a button and then the Pressed version of it.

Now im wondering about the following:

1. How do i use the Rollover Effect? i would like to use the minimal possible code to do so.
2. How do i position the .png button images? Edit: i also noticed that if i place a png file as my background for every of my 3 pages, that it allways flickers when the new page gets loaded, the background is not a simple color but a circular gradient, i know that it is possible to hardcode a background color in html but is it also possible to do a circular gradient background?

is that what CSS is for?

also i read about definite and relative position and im wondering where i can i learn to do it also at a later point i would like to learn how to do loading bars or circular loading bars (if i check off itmes on my To Do list the loading bar gets fuller, i read about frames etc I really would like to learn it at a later point.

I think i went waay ahead of my self and im going back to the class and do the exact thing. But im wondering if i learn those in a later point and where to read about it.

Sorry for so many newby questions! Thank you!!