Sidon is the proper translation for テーマ since it's katakana (mainly for foreign sounds) they use "Shi" to represent the first syllable. They dont have "Si" in their alphabet. Since all the media it's referred to Sidon, I'll keep that one and the rest for the tags.
This will be the last thing i'll fix because i have to check the offset and such. I don't want to screw it up