Image Dropzone background isn't proper with the oxide-dark theme

[advayumare]

What is the current behavior? Describe the bug

The Image upload dropzone has a white background

Please provide the steps to reproduce and if possible a minimal demo of the problem via fiddle.tiny.cloud or similar.

What is the expected behavior?

The background should be darker and aligned with the dark theme

Which versions of TinyMCE, and which browser / OS are affected by this issue? Did this work in previous versions of TinyMCE?

TinyMCE version: 6.0 Browser: Google Chrome Did this work in previous version? NO