Booster theme subscriber input change to underline

[minimalanimal]

Hoping someone can help:

I am currently using the "Booster" theme and would like to make a change to my Newsletter Subscribe input field and button. I've adjusted the button from outline to underline, but since I cannot adjust the input field, it looks odd.

At the moment it looks like this for me (also, the left alignment is off):

 

 I would like it to look more like this:

Can anyone tell me where and how to edit the code to change this? I'd be very grateful.

Many thanks!