A dark colored grout, but it has to be done very carefully and the joints cleaned and shaped to make thinner grout lines than typical.

The mosaic installation is indeed lacking. The wall tile, there are usually pattern restrictions when installing tiles that are long like yours, if they are over 12" in length. The reason is that often times these tiles are bowed, they are not perfectly flat. Take two of the tiles, stack them so glaze is touching glaze. Do they rock against one another? Installing these tiles in an improper offset pattern can result in the "high midsection" of one tile being next to the "low end" of the neighboring tile. Lippage. Grout colors, if powdered grout was used, I'll dry mix all the powder first, then portion it out as I mix batch to batch. That way you don't suffer from color changes due to differing lot numbers. Most color changes come from water. Too much water used in this batch versus the other batch, too much water used when cleaning up this section versus the other section, etc. Some color variation can blend out over time once all the grout and thinset is equally cured. The grout can wick moisture from the thinset behind the tile. Even slight differences in moisture can cause grout to shade differently. Acid washing? That can kill the color as well. I never use acid on cementitious grout. It kills the color and the texture, who knows what dilution rate was used. Definitely bring in supervision. You have issues. The mosaic is bad. If the wall field tile is large format and the manufacturer has published restrictions on how is should be installed, perhaps those instructions were not followed. Basic TCNA installation instructions not being followed, caulk versus grout used in the changes of plane, for example. When I see sloppy finish work, it does make me curious about the attention to detail given to the waterproofing behind the tile, the stuff that can't be seen. Good luck getting this resolved to your satisfaction.

I think a plain cream panel with a hefty fabric will look nice. You already have cream accents in your space and they will look nice with the green walls. If you can get the colors right, a green print would look nice as well and retro/modern. This graphic has good curtain advice. High, full and wide is best but don't cram them against the ceiling. Remember that you have to work with stock curtain lengths. You can add a bit of length to curtains if you hang them with rings.

I think dark brown grout would make the tile colors pop.