Like summoning reagents but for Flame Weapon/Darkfire. Ichor for acid damage, icedust for cold damage, that sort of thing. It would allow tailoring of the spell to get around creature immunities etc.
This can already be earned in game by mages. Whether the theme is switchable at will is something that I don't know.
I know it can be asked for at creation or after, but it's not switchable, it represents a PC choice to focus on that sort of power (like a summon theme). That's why I suggested this as an idea.