Changing height of header

Did you mean max-height: 5px; ?

Try:

header.tc-header {
max-height: 5px;
}

but will need a bit more when I understand what you’re trying to do.

Doh! Thanks. However, it still doesn’t reduce it by the amount I want.
Check out:
https://nzgs.rsnz.org/wp/aec/
I’m trying to get it so the white area at the top is gone or at most a few pixels.

Easier to make it:

header.tc-header {
display: none;
}

Done. Many thanks!