You're right about the 1, 4, and 5 being major, but that applies to major keys. What it really i means is that in any given major key (And I mean ionian, based on the one chord, not based on mixolydian or lydian) the fourth and fifth chords will also be major triads (the fifth chord is actually a dom7 chord when you extend it though.)
As for learning theory, there's a lot less stuff to know than most people act like there is. I mean by no stretch do I know all that I'd like to know, I'd like to be more familiar and comfortable with some jazz progressions, I still am not used to the conventions behind the chord progressions of people like thelonius monk, who did some incredibly interesting complex stuff.
I'm going to recomend a site I just found to you, and to everybody. For me, it's probably the most useful guitar site I've found on the web, at least in terms of learning stuff. It's called http://www.jodyfisher.com. I have two books by the guy (jody fisher) that i got at a summer guitar workshop, and looking at his site and the books I realize this guy is a great teacher. It has lots of theory, and lots of ways to apply it too. If you are at all interested in playing jazz you should check it out (it's mostly a jazz site.) If you like playing latin music, it has 25 different written out rythyms that you can use to play chords in a latin style music. If you're trying to work out voicings for a certain progression, it has 108 (!) different ways to voice a ii V I in G major. Along with this it has great theory stuff.