I’ve just downloaded that theme for a quick look and there’s absolutely no mention of any content-page.php file nor a content folder in the downloaded theme. where did you get the code that you added to your custom page template? Did you realise that any example code in the Codex here is just that – an example. You have to modify it to suit your theme.
For that theme, I’d try creating a copy of the theme’s page-custom.php file, renaming it ad amending its name in the content block and then start modifying this new file.
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