Copy the Oxygen folder in your /style/, name it eg. Mytheme.
Next rename files Oxygen.php and Oxygen.min.css, to eg. Mytheme.php and Mytheme.min.css.
Now You should have structure like this
style/
---Oxygen
---Mytheme/
------feed-icon.png
------index.html
------Mytheme.min.css
------Mytheme.php
Last thing, you have to edit Mytheme.php
$forum_loader->add_css($base_url.'/style/Oxygen/Oxygen.min.css', array('type' => 'url', 'group' => FORUM_CSS_GROUP_SYSTEM, 'media' => 'screen'));
to
$forum_loader->add_css($base_url.'/style/Mytheme/Mytheme.min.css', array('type' => 'url', 'group' => FORUM_CSS_GROUP_SYSTEM, 'media' => 'screen'));
Now you can choose Mytheme in your profile as your style. Last thing is to edit CSS of course .
If any of my dropbox link fails with 404 error, change dl.dropbox.com/u/56038890/punbb/*.zip in address to 82283017.
Currently working on rPlus - responsive theme for developers to create their own themes basing on this one.