Creating Mashups is a great way to give your visualizations a nice looking container in the form of a custom HTML web page.
Inserting custom-built extensions into your mashups, however, requires one extra step compared to standard objects.
First, custom extensions can be added to mashups just like any other object. However, the custom CSS file needs to be added to the HTML page of the mashup.
Let's assume both your extensions and mashups are stored in this folder (they will be by default):
You'll need to add this line in the <head> of your MashupName.html file: