Booster theme subscriber input change to underline

Topic summary

Goal: restyle the newsletter subscribe form in the Booster theme so the input field matches an underlined button and fix left alignment. Current state: the button has been changed from outline to underline, but the input field remains with its default outline, creating a mismatched look and misaligned left edges. Desired outcome: an underlined input field and properly aligned elements, as shown in the provided reference image. Request: guidance on where and how to edit the theme’s code (likely CSS/HTML) to apply an underline style to the input and correct alignment. Notes: screenshots are central to understanding the visual target. Status: no solutions or decisions yet; the thread is an open troubleshooting request.

Summarized with AI on December 30. AI used: gpt-5.

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!