Support dark theme

[isabela-pf]

When the theme is switched to JupyterLab's stock dark theme, there are a few parts of the interface conflicting with the switch.

1. The text formatting icons do not switch to their light color
2. A white box flickers in the background of the cell when it is first selected.
3. Dropdown menus do not switch to the appropriate dark color.

@bollwyvl has already started on issues 2. and 3. in #35. (Thanks again!)