The accent color should be used for interactive elements

In the Material Design Style guide the accent color has the following definition:

The accent should be used for the floating action button and interactive elements, such as:
- Text fields and cursors
- Text selection
- Progress bars
- Selection controls, buttons, and sliders
- Links

In MDL, it appears that the primary color is used for this. The switch color,
for example, is defined with the following value:
`$ switch-color: unquote ("rgb (# {$ color-primary})") !default;`