Mobile app visual - change subscribe button to 'solid' on top bar

Summary

(Summarize the bug encountered concisely)

As of now, the subscribe button on the top bar looks harsh in certain contexts. Make it so that the button is solid in all contexts, which would be able to work with both a "dark" as well as a "light" banner.

Steps to reproduce

(How one can reproduce the issue - this is very important)

Platform information

(Browser, device, system stats, screen size)

What is the current bug behavior?

(What actually happens)

What is the expected correct behavior?

(What you should see instead)

Relevant logs and/or screenshots

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Possible fixes

(If you can, link to the line of code that might be responsible for the problem)