Non consisten color name

[lchrusciel]

Hey Maciej,

thanks a lot for the whole course and your work.

I have spotted a small issue. When I've downloaded your project background of the project was black, while I expected a violet one. Nevertheless, default implementation still suggests a violet as a variable name:
https://github.com/maciejkorsan/wtf-gulp-starter/blob/master/src/scss/_variables.scss#L1

I'm aware, that issue is really minor, but on the other hand, it can be misleading, especially for the newcomers.

I can open a PR with a fix, but just I'm not sure if the variable should be named black, or color should be adjusted.