Fix "Dropzone" coloring in dark theme

Looks like the text in the "drop-zone" automatically changes to the "correct" white color, but the background doesn't. We'll want to make the background shift to a more appropriate color. I'm thinking some shade of grey so it's different and unique from the surrounding area. Will need to further explore options.

The dropzone is made using material-ui-dropzone a package available on npm. There's documentation resources here:

• npm: https://www.npmjs.com/package/material-ui-dropzone
• GitHub: https://github.com/Yuvaleros/material-ui-dropzone#readme

There must be some way to interact with the dropzone to use the theming, maybe we just need to set some more of those color options as we saw in Material UI's Dark Theme Docs