WowAce

Sorry for asking a question under issues, didnt know where to turn really.

I have my Themes set as following. 

But when playing Heroic the themes dont really follow the raid size, like, often we can be 18 people in the raid but the theme for 30 players is enabled. I understand that its hard to determine what theme should be enabled so just looking for guidance on how to set up the themes.

Grid2 uses the instance raid size (the max raid  size is defined by the game for each instance+dificulty) , not the actual numbers of players in the raid group, for example a mythic raid instance is always a 20 man raid instance, a normal raid instance is a 30 man raid instance, etc.

Any chance an option to use number of players could be added?

No, changing layout/position/sizes, are expensive and restricted operations that can't be done often or in combat.