...will they overheat plant roots? There will be filler of some sort in the beds, for ex. mulch or bark. So the stones won't be directly against roots although I still am concerned about burning the roots. I'm also unsure what is the most attractive method to use. I have enough purely gravel fl. beds elsewhere. I want to soften the appearance.

It shouldn't affect the soil temp at all! Pebbles work like a mulch when used as you described.