Thanks for getting in touch. Custom CSS like this is not yet possible with TubePress Express. We are actively working on that capability, but for now you’re limited in Express to the themes that are available under the Theme tab.
You can, of course, use custom CSS with our downloadable WordPress plugin 1 by either adding it to your WordPress theme’s style.css or by creating a custom TubePress theme 2. I’m not sure if that is a viable option, but please feel free to contact us if you have any further questions!
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