Making slabs and stairs that tint with the biome

Grass blocks are only partly tinted- the dirt texture is always the same, but the grass portion changes color based on the biome. For regular blocks, you would specify an overlay texture to go on top of a non-tinted base; but for slabs and stairs, that'd be more difficult.

...You can make custom blocks have biome tint though, so if you really want this feature, you could make your stair/slab blocks in Blockbench, and assign biome tint to the grassy portion of the texture. However, this might be more trouble than it's worth, as you'd then have to replicate all of the other properties of slabs and stairs. (Namely, getting them to change their shape appropriately.)

It's just the rotation bit that's the issue. If you just wanted slabs, for example, that'd be a lot easier because you only need two variants. (Top/bottom.) Stairs are just tricky because there's at least sixteen different configurations.