Padding not working?

Question

css

In general, I made a layout and uploaded it to github
https://github.com/zarnyenn88/zarnyenn88.github.io
(zarnyenn88.github.io)
Can you please see my site? I don't know who else to ask about it. Tell me what you did wrong. At the end, it has not yet been possible to make a normal "subscribe" button, the bottom padding does not work. That is, you need to have the same (same color) bottom border of 20px as the main block. But he is pure white for some reason. How to fix?
And what do you recommend in general? Perhaps BEM and flex should be studied in more detail?