Button hover colors should be in _variables.scss file

Created by: mwentz

The mixin for button-variant is defined like this:

@mixin button-variant($background, $border, $active-background: darken($background, 7.5%), $active-border: darken($border, 10%))

Because of this, if you want to have the active background 10% darker or 7.5% lighter (for example) you either have to change the mixin file or write more custom css. This could be easily fixed:

/mixins/_buttons.scss

@mixin button-variant($background, $border, $active-background, $active-border)

_variables.scss

$active-background: darken($background, 7.5%);
$active-border: darken($border, 10%);

It looks as though this was probably the original intent, but somebody forgot to move those variables out of the mixin file.