Allow custom styling / themability of widgets

[batpad]

Started sketching out some ideas in #1, but this needs a bunch of thoughtfulness applied.

We need to

• do a bit more background research on how theming works in both JupyterLab and MyST
• define common CSS variables and styling patterns to make widgets themable in a consistent way
• define consistent patterns when dealing with 3rd party libraries (eg. ChartJS) to have theming propagate down
• allow setting of themes to widgets (or at a higher level like a global theme / applied to layout components and propagated to child widgets, etc.) and / or over-riding more granular style properties
• add documentation on creating widgets to be easily themable as well as creating new themes and applying them

This definitely seems possible to do, but is the kind of thing that's really hard to do right. @dzole0311 really appreciated your thoughts in #1, will be great to get your help thinking this through and what the best interface should be.