Hiking trails in some areas are very faint. Is there any way to make them thicker or change the colour?
I’m presuming that you’re talking about OSM’s “standard map style” here. That’s essentially been optimised so that well-mapped central European cities look OK, at the expanse of more sparsely mapped areas. See https://github.com/gravitystorm/openstreetmap-carto/pull/747 for some of the discussion around that.
It’s not actually possible to have one map style work “everywhere for everyone”, and to have it both “look nice” and “provide mapper feedback on everything added to OSM” - any map style has to be a compromise.
What I did was essentially to stop using the standard map style and start using my own. Depending on where you are, there may be other public OSM-based maps that make a better job of hiking paths. If you can’t find one, it’s not that hard to create your own - instructions for creating a “slippy map” using my map style are at https://wiki.openstreetmap.org/wiki/User:SomeoneElse/Ubuntu_1604_tileserver_load , and for creating one using the “standard” style https://switch2osm.org/manually-building-a-tile-server-16-04-2-lts/ .
Sometimes a picture is worth a thousand words so compare http://www.openstreetmap.org/#map=14/53.1731/-1.0334 with https://map.atownsend.org.uk/maps/map/map.html#zoom=14&lat=53.17651&lon=-1.04135 . Also it’s worth mentioning that “Waymarked Trails” will work globally and will highlight hiking paths that are part of a relation: https://hiking.waymarkedtrails.org/#?map=14!53.179!-1.0337 .