How to shrink sticky header (V1)

Hi,

My header doesn’t shrink when I scroll down, and it takes way too much space. I tried to reduce just the size of the logo, the header itself, but the sticky header stays too big when I scroll. Also, I like having a sticky header, so I’d be happy with keeping the V1 header.
Which CSS can I add to shrink the V1 header when scrolling? I have the free version of the agama blue theme ^^

Thanks!