Using the Extension Manager, install the Advanced Configuration Plugin:
https://www.dokuwiki.org/plugin:advanced.
Select
Style->Screen from the menus. This will let you add entries to conf/userstyle.css. Add the following:
pre.code {
background-color: white;
border: 0px;
box-shadow:none;
}
Then click Save and Purge CSS Cache. Then purge your browser cache.