If you want to change the text of the login page, see
https://www.dokuwiki.org/localization#changing_some_localized_texts_and_strings_in_your_installation
The text is stored in an own file.
It is also possible to change the look with css:
https://www.dokuwiki.org/devel:css#user_styles
If you want to change the php code of the site, you will have to hack the code, but those changes won‘t survive an update.
The code for the search is located in /inc/search.php