Describe the bug
A clear and concise description of what the bug is.
The dropdown menu on the global types that are defined in the menu is displayed with black text on a black background when using a Mac with dark theme.

To Reproduce
Steps to reproduce the behavior:

1. Install the Storybook a11y add-on or define a global type
2. Set Mac to dark theme
3. Click on one of the menus in the toolbar
4. See that you cannot read anything

Expected behavior
A clear and concise description of what you expected to happen.

The menu should not change background color when dark theme is set, or at least the text should also change color.

Screenshots
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This one struck me as particularly ironic:

Code snippets
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System
Please paste the results of npx sb@next info here.

% npx sb@next info

Environment Info:

  System:
    OS: macOS 10.15.7
    CPU: (8) x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
  Binaries:
    Node: 12.21.0 - /usr/local/opt/node@12/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.11 - /usr/local/opt/node@12/bin/npm
  Browsers:
    Chrome: 89.0.4389.114
    Safari: 13.1.3
  npmPackages:
    @storybook/addon-a11y: 6.2.2 => 6.2.2 
    @storybook/addon-actions: 6.2.2 => 6.2.2 
    @storybook/addon-essentials: 6.2.2 => 6.2.2 
    @storybook/addon-knobs: 6.2.2 => 6.2.2 
    @storybook/addon-links: 6.2.2 => 6.2.2 
    @storybook/addon-postcss: ^2.0.0 => 2.0.0 
    @storybook/addon-storysource: 6.2.2 => 6.2.2 
    @storybook/addons: 6.2.2 => 6.2.2 
    @storybook/react: 6.2.2 => 6.2.2 
    @storybook/source-loader: 6.2.2 => 6.2.2 
    @storybook/storybook-deployer: ^2.8.7 => 2.8.7 
    @storybook/theming: 6.2.2 => 6.2.2 

Additional context
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