OSM: Don't render administrative boundaries

I have created my own OSM server to pre-generate tiles for later offline use in a desktop app I’m currently coding (I save the tiles in a local SQLite db).

Since I followed blindly a tutorial found online, I’m not sure how to modify what is rendered on those tiles : I would like to have no administrative boundaries shown on the tiles, except country borders.

Right now I have counties, departments, regions etc boundaries and sometimes it’s quite hard to differentiate them from paths in the woods.

How can I do that?