I'm currently learning music theory and got a grasp of its basics but I want to learn more. What are some music theory topics that I should learn to become a better musician and guitarist?
I would suggest learning chords, scales, and arpeggios first
Chords: Major, minor, diminished, Dominant, Major 7th, minor 7th, diminished 7th, half diminished.
Chord families (Tonic, Subdominant, and Dominant)
Learn them in different positions and inversions. Play them as arpeggios and as block chords.
Scales: Major scale and its modes (Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian, Locrian) and their construction.
Play them up and down, in broken thirds, arpeggios, etc.
Harmonize scales by thirds and learn the chord quality of each degree. Each degree of the scale has a specific chord quality and mode.
Here are the degrees of the major scale.
( I - ii -iii- IV- V- vi- vii° )
Once you mastered those scales, you should learn the modes of the harmonic minor and melodic minor scales. Then, the symmetrical, diminished and whole tone scales. It's also important to learn the context in which these scales are used. Most of the time they are played against certain chord or chord progressions.
Play everything in all keys and be aware of what notes are being played. Learn the circle of fifths, it will give you a better understanding of how keys are related and organized.
Learn about secondary dominants and modal interchange, then choose a jazz standard and try to find examples there- believe me, in almost each jazz standard there are many.
I recommend to check some of the lessons on the site, there is a lot of great theory stuff, and explained in simple terms. I hope this is helpful to you!