See this thread:
https://forum.dokuwiki.org/thread/11931
Then it's just a matter of finding the id or class of the element(s) you want to hide and set them to { display: none; }
For example, this is what I have:
.mode_login #dokuwiki__header,
.mode_login #dokuwiki__footer,
.mode_login #dokuwiki__content h1 {
display: none;
}
.mode_login,
.mode_denied {
#dokuwiki__content p {
width: 640px;
margin: 2em auto;
text-align: center;
}
#dokuwiki__content {
text-align: center;
}
}
Line 6 and onwards is just for styling the login page box, etc. Hoping this helps!