page_theme.php not working in my custom theme2 users found helpful
When creating a theme, in description.txt, be sure the name of the theme on the first line agrees with the name of the folder the theme is loaded in (e.g. in description.txt use "Hornet" for the theme that's loaded at application/themes/hornet. Do not use a non-matching name like "My Beautiful Hornet Theme" in description.txt. Use all lower case characters for the theme folder name.
If the theme name includes spaces, represent these as underscores in the folder name. So in description.txt the theme named "Green Hornet" should be in folder application/themes/green_hornet.
When using page_theme.php to set up awesome customizations for your theme start with a simple file (example below) or copy the file from one of the included themes in concrete/themes and remove the functions and features you don't need.
IMPORTANT: change the namespace line at the very top of the file so it matches your custom theme. For our example it would look like:
When your theme name includes a space namespace line should be CamelCase [thank you jakobfuchs]:
Be sure nothing appears before the namespace line, aside from the <?php line.
Finally, if you are adding page_theme.php to an existing, enabled theme you can't just upload it and expect it to work. On your site enable a different theme, REMOVE the theme you're adding page_theme.php to (use the red Remove button), and then, with page_theme.php now uploaded, install the theme again and reactivate it. That will tell C5 that page_theme.php is part of your theme. This has been mentioned elsewhere more than once; I'm just including it here for compleness.
Hope this helps.
I corrected my post to indicate camel case for the theme name. Thanks!