Depending on your template, you could add something like this to your css (remove lines as necessary):
.mode_login #dokuwiki__header,
.mode_login #dokuwiki__footer,
.mode_login #dokuwiki__pagetools,
.mode_login #dokuwiki__content h1,
.mode_login #dokuwiki__content p,
.mode_login #dokuwiki__content .docInfo {
display: none;
}
.mode_login #dokuwiki__content .page {
position: absolute;
margin-top: 20%;
margin-left: 50%;
left: -240px;
}
Disclaimer: The above code was just a quick hack for the dokuwiki template, and not tested across the board, especially in terms of positioning and size. Please implement at your own discretion. :)