I think this is true in Disney Canon and legends yes, I believe Qui-Gon Jinn also harnessed this sometimes but I could be wrong

Jedi don't harness their dark side, they keep it under check, so they can stay in balance, i.e. staying on their light side and not letting their dark to conquer them. In truth, Mace Windu has a purple lightsaber because Samuel L, Jackson wanted a purple lightsaber, and George Lucas decided to go with it. In Legends, the Force is treated differently than in Lucas' own works, so I don't know much about that, but I don't think the purple color was meant to signify something like this in those stories either.

^ Surprisingly, this may be one of the few times you're wrong about something!

Mace Windu does in fact tap into the Dark Side when he fights, hence his use of Form VII, Vaapad.

Well, more specifically, he harnesses his opponent's Dark Side power.

The purple lightsaber thing is mostly because Jackson wanted a purple one but the in-universe explanation is as abovd.

His fighting style vapaad relies on him being able to use the dark side against his opponents without giving into it.

That's definitely true

Well, look at that! I didn’t realize they made that canon. But remember, I rarely go beyond Lucas’ Star Wars story.

@Dat1899 Qui-Gon actually didn't.