Try to keep these maximally compatible with the default sets. I believe this is supposed to get easier when the new formats arrive, but since order is still dependent, your object order should match the defaults. If I change the set the level uses (by renaming whichever variant of "dirt" is in use, then selecting a different one when the editor asks for a replacement), the color palette should be the only observable change.
Considering the background issue, I have created a fixed version containing the following changes:
- The object ordering and naming now matches dirt and dirt_md
- The background has been changed to 0,0,0
- Mask/bridge color changed to 255,255,85 (my screenshots suggest 252,252,84 but this should match a color on the terrain and the terrain in this set uses 255,255,85. This is within reasonable error caused by emulation, however, and should probably not be messed with further). Reasoning for this change is shown in the attached screenshot.
- Fixed a terrain piece that was not recolored properly. Seems the orangey parts got recolored but the gray ones did not for some reason.
Finally, as an organizational note, I'd suggest keeping a collection of the styles somewhere you can download them all at once - perhaps a .zip with all of them? That can wait until they're all done, though.