that is no longer necessary (I have now also removed the instruction from the description).
simply subscribe and enable it ingame (with the lowest priority/top of the list)

How do you change the base map to grey?

我製作了一個複製 OSM 的mod。這就是你要求的嗎？

長時間看灰圖對眼睛實際不好... 有辦法 鐵路顏色套用 OSM嗎? (使用ORM配色對OSM底圖)

I'll fix it in the next update. It's gonna be a total overhaul of the mod, and much more true to the real ORM.

how do you make the base color gray

Yes I am, this was admittedly a rushed version of the mod, and I'm currently working with L8Purple to make a proper OSM MapStyle mod, once that is done I will update this mod to be equally as accurate.

Loving this map style, but could you have a look at the stroke width/colour of bicycling paths? Playing on a Dutch map, these show up a lot, and are marked with a rather heavy black line, rather similar to many rail pieces. ORM also shows these in black, but uses a dashed line. Personally, I'd prefer a grey (dashed) line similar to footpaths.