Changes description of mixin - props @r007

Relates to #582
Rewording "plugin" to "theme"
Merge pull request #586 from yoarts/patch-1

Change plugin references to theme references in Travis CI config file.
Add fixes for faulty Sass @import paths and negative values.
Change Sass headings file name and import path.
Merge pull request #596 from MickeyKay/sass-header-file

Change Sass headings file name and import path.
Merge pull request #594 from MickeyKay/sass-fixes

Add fixes for faulty Sass import paths and negative values.
Closes #588.