Hi,
Thank you for all the details!
I logged in to check the configuration, and I think the issue was only the input field background. It was also set to red, and with that theme the input field covers almost all of the search background space. I changed that option to transparent, now the change should be visible: https://i.imgur.com/WmlxJeQ.png
I also quickly changed the magnifier background color, to match the search bacground for you.
I will check if that input color is set to transparent by default on all themes, to avoid this issue, I may have made a mistake somewhere, as it should be transparent by default.