MessageSpace scroll button is all white on dark mode

[kaseyvee]

Describe the bug
The button that allows people to scroll to the bottom of the MessageSpace component is all white on dark mode, so no text is shown.

To Reproduce
Steps to reproduce the behavior:

1. Go to the MessageSpace component on storybook with the theme set to dark mode.
2. Scroll up in component until white button with no text appears.

Expected behavior
There should be text in the button that tells people to scroll down.

Screenshots
Light mode:

Dark mode (bug):

Please complete the following information:

• Browser and version: Chrome version 124.0.6367.210 (arm64)