Removed backgroundIn animation and cleaned up the color vars.

I wanted to cleanup the color variables so that the Artists Theme user only have to specify one accent color. 

First i tried to generate the 'accent-light' from the 'accent' by using bourbons tint mixin. But in order to use it I had to import the '1-tools/-tools-dir' before everything else. That caused a problem in backgroundIn because it used a variable. I removed backgroundIn because it had the same effect as fadeIn. In the end I didn't use the tint mixin but the '1-tools' shouldn't require any variables to work. So, that's fine...

Now the user only have to specify one accent color and every other color gets generated automatically. '$linkColor' is now also '$accent'. 

The end result has no difference but it is a lot easier for the user. :smiley: