Would be good to align all tooltip styling

## Description

There are different tooltip styles for the plot controls, in the 3D Viewer viewing area (dropdowns), and in the left panel. It would be nice to chose just one style that has sufficient contrast on any background.

Related to:
https://github.com/allen-cell-animated/website-3d-cell-viewer/issues/53

## Expected Behavior

We should align tooltip styles and just choose one.

## Reproduction

Hover over areas mentioned above in CFE

## Environment

n/a

## Notes


This is the image of my suggestion which I've already outlined in the style guide master file here for CFE:
https://xd.adobe.com/view/41f74a61-2a1a-401e-b824-1aace1f98e2e-14a4/

font-size: 12px;
color: #fff;
background: #595959;