General theme support

[jeremy-moffitt]

Understanding that Octant is not intended to have full white-label support, it would be useful to have some level of uniform theme or branding support. Clarity already supports sass overrides, and there is a dark theme issue (#36) that perhaps the community could use as a reference point? The ideal result would be some official best practice that allows for a basic color swap at a minimum without needing to fork the repo just to add branding. This could be delivered as guidelines, or a default theme that can be overridden.

[eapodaca]

I've taken some time to look at this and run a few experiments. I think the only thing that would be nice to have for this are some changes to a few styles.

There are a few instances where colors are in a few component specific scss files. It would make our theming effort more simple if this was done though is not 100% required.

[eapodaca]

It might also be nice to enhance the plugin mechanism so that plugins can specify additional styles and or assets at runtime.