Hi Tom,
I am happy that you are enjoying 3D viewer.
Unfortunately, at moment, there is any kind of (images) texture load for 3D Viewer.
I am not sure also if STEP supports textures at all.
However, depending on what you consider “textures” there could be some ways to get it displayed in 3D Viewer.
Have a look on my answer on this thread:
The idea is to “vectorize” what you consider the “texture” and convert it to a 3D shape (ideally non solid, for performance)
So most of the things like text on ICs or packages in 3D are created on some kind of this way. (extruding+add or extruding+subract)