As a modder, aesthetics is everything. It happened that I completed a big project without even installing Windows. In a personal pc situation, i think it takes about a good 70%
When I was a teen, they mattered. Honestly these days I'm all about function over form, my case sits under the desk and out of sight. Then again I just bought a Razer chroma mouse pad ...
Alot. I usually pick a few colors for a build and stick to them. Luckily everything is RGB these days so I can change the color or whatever parts I need!