Fix custom themes breaking the welcome demo.
Fix custom themes breaking the welcome demo.

Having applied a dark theme to storybook I noticed that the react demo sets the background white but does not set the text color. This resulted in white text (from my custom theme) rendered on a white background. This change will make the text readable
storybooks/storybook,storybooks/react-storybook,storybooks/react-storybook,kadirahq/react-storybook,storybooks/storybook,storybooks/storybook,storybooks/react-storybook,storybooks/react-storybook,storybooks/storybook,storybooks/storybook,kadirahq/react-storybook,storybooks/storybook,storybooks/storybook