There are lots of websites out there that help to build your music theory skills. Here's a few to try out....
MusicTheory.net - with free lessons in all sorts from notation and rhythms, to chords and intervals. With interactive exercises that keep your scores and make it fun.
My Music Theory - free online courses with tutorials and tests, aimed at the ABRSM theory grades.
Teoria - more interactive theory and listening exercises, plus tutorials. Also covering musical form.
Interactive Circle of Fifths - find out how modes relate to the circle of fifths....
And for more advanced studies....
Ian Ring on Scales - there's a lot more to scales than meets the eye (or ear), as you'll find out in this more academic study.
Jazz piano theory - a detailed look into the theoretical concepts and improvisation practice underlying jazz piano. With lots of YouTube tutorials too.