The terminal plugin in Code follows the settings of the main Terminal application. To edit these settings:
Close Code and open Terminal.
sudo apt install dconf-editor, then close Terminal once the package is installed.
Open dconf Editor from the Applications menu.
You can now edit any of these settings by clicking on them. If you're interested in changing the colors of the interface, you'll probably be most interested in background, foreground, and palette. Background is obviously the background color, foreground is the main text color, and palette is the colors of various other features (this might require some trial and error to figure out what each of them actually controls).
Note that to get these changes to update in the Code plugin, you may need to Disable it and Enable it again in Code's settings.