I thought the theme was fine. The geometry is lower secondary school maths, the associated name and place well known and the Greek alphabet is common knowledge. The only thing I did not know was the message, but Wikipedia readily supplied that, and it was cleverly done.
What I disliked about this crossword wasn't the theme but some of the clues. I particularly dislike clues that require reading entries in Chambers for particular wording. Rather than the wordplay leading to the answer, you almost have to know the answer before you can understand the wordplay. 13 down, as previously mentioned, illustrates my point perfectly.
For me, the best setters are those who use common English meanings but in a way that challenges your interpretation. That's particularly true in wordplay where you have to identify the redundant words to be removed. In such puzzles, the parsing should be crystal clear.