If you have the right perspective the good candidate moves fall out of matrix of variations like ripe fruit. 

Take for example the position in the diagram you posted and I quoted. 

1. After the move 6.g3 as Black I would instantly think of the St. George Attack if White castles on the Kingside (0-0).

2. If White plays Bg2 he has lost the possibility of playing the thematic Bc4.  However, he does get the limited diagonal h1-f3 and he is exerting additional pressure against the thematic d5 square from behind the White pawn at e4.  This makes Black's eventual ...d5 break more problematic if Black even finds it necessary to play ...d5.  Additionally with the B at g2 White forfeits all tactical shots at e6 in combination with the N at d4 in case Black plays ...Nbd7 at the wrong time.  I have, however, reviewed a book by IM Tappani Samualvo on this 6.g3 idea, where White plays Bh3 but only after h4 and the tactical shots at e6 come to life again from the diagonal h3-e6 instead of b3-e6.  I have also read an article about IMSamualvo's book by GMJoe Gallagher that is critical of Samualvo's analysis.

If you would like to know more, and more about how pawn structure, the Hypermodern and Siege Warfare (restrain, blockade, and execute the enemy) combine to produce a perspective on any position that causes the candidate moves to fall out of the matrix of variations like ripe fruit, please let me know.

I really don't think of Bc4 as a thematic move in the Sicilian.

During Bobby Fischer's time he popularized 6.Bc4 to a point where it was thought of in some chess circles as thematic.  Today of course it is recognized that this is not the case