The default theme of the system can be overriden by means of a custom theme file in SCSS/themes. The custom theme file should re-assign values to variables, for example:

$background: #333;

The above rule willl override the initial value given to $background. Custom theme files should be included in scss/_build.scss for it to be included in compilation. For global variables to be allowed to be overriden, they are initially declared using the default parameter:

$background: #fff !default;