Use the colour variables defined in _variables.scss

[robyngit]

The _variables.scss file defines colours that are commonly used throughout the site. However, some .scss files (especially under /partials/) use the hard-coded hex colour code instead of the variable.

It's better to use variables rather than the colour hex-codes where possible (makes it easier if we want to change the colours one day.)

[ThomasThelen]

A took a stab at this in 217e4ed . Note that some thing weren't changed. For example gradients

I went though all of the files except the ones in data-viz. I'm not sure if you want me mucking around in those styles, but I definitely wouldn't mind

[robyngit]

More progress made on this issue in #126, though there are still hex codes that should be switched to variables, and color variables that could probably be combined. Since everything works and looks good though, I think this issue should go on the back burner to be fixed if needed in the future.

[ThomasThelen]

Fixed in #136