will you get banned for using custom skins/particles ?

If Riot finds out you're using third-party software to modify ingame graphics, yes, they might ban you.

Custom skins can be a way to cheat. Just know that even some Riot skins are banned in pro play due to conferring an advantage upon those using them.

Riot's official stance is that it's only forbidden if it either gives you advantages in any way or allows you to use Riot's skins that you don't own. They even went on to encouraging people with arachnophobia to use custom skins for Elise for instance.

Riot said no, they won't ban for custom skins. Just don't use a custom skin for a skin you want but won't buy. Like a little devil teemo "custom skin." That's stealing. Also, they said that while they won't ban, they also make no promises and that can change and to use at your own risk.