Where does “θεματικός” come from?

θεματικός (Ancient Greek) comes from Ancient Greek θέμα, from Ancient Greek τίθημι, from Proto-Indo-European dʰeh₁- — to do, put, place.

Ancestry of “θεματικός”, step by step

Step	Language	Word	Meaning
1	Ancient Greek	θέμα	That which is placed or laid down; Something...
2	Ancient Greek	τίθημι	I put, place, set; I make, cause to be; I make
3	Proto-Indo-European	dʰeh₁-	to do, put, place