Just found this thread, there is soon a working Step/One driver in MAME, for the next release we have support for 320x200 8 color mode and 640x200 b/w mode right now working on the 640x400 interlaced mode which I hope to add as well. Below is a couple of screen shots. Cred goes also to Mattis for the disk images and Fredrik Öhrström who has written the keyboard driver and provided BIOS dumps.
Finally got the Step/One "feature complete" merged, there was some problems where the service manual was not describing a graphics mode correct, it turned out that the machine indeed supported a high res color mode, aka 640x200 in 8 colors, I got evidence from Weetmuts who owns a physical Ericsson step/one (a branded Mybrain 3000) so I had to figure out how that worked and I found that the hardware uses 8 bits for green and duplicated 4 bits each for red and blue to form the palette index for each pixel. This means that any color involving red or blue will affect at least two pixels in width. Green is ok for single pixel lines however.
Here are some screen shots from some of Weetmuts step/one floppy dumps: