Can't use demo data with other templates

[Sogl]

Hi!

I use grav one day. I have installed your theme by bin/gpm install medium with demo data and it works fine, but when I change it to another (Antimatter, for example), I see an error:

RuntimeException thrown with message "Template "post.html.twig" is not defined."

Stacktrace:
#9 RuntimeException in /Users/Artyom/domains/gravru/system/src/Grav/Common/Twig/Twig.php:345
#8 Twig_Error_Loader in /Users/Artyom/domains/gravru/vendor/twig/twig/lib/Twig/Loader/Chain.php:115
#7 Twig_Loader_Chain:getCacheKey in /Users/Artyom/domains/gravru/vendor/twig/twig/lib/Twig/Environment.php:312
#6 Twig_Environment:getTemplateClass in /Users/Artyom/domains/gravru/vendor/twig/twig/lib/Twig/Environment.php:378
#5 Twig_Environment:loadTemplate in /Users/Artyom/domains/gravru/vendor/twig/twig/lib/Twig/Environment.php:347
#4 Twig_Environment:render in /Users/Artyom/domains/gravru/system/src/Grav/Common/Twig/Twig.php:334
#3 Grav\Common\Twig\Twig:processSite in /Users/Artyom/domains/gravru/system/src/Grav/Common/Grav.php:186
#2 Grav\Common\Grav:Grav\Common\{closure} in /Users/Artyom/domains/gravru/vendor/pimple/pimple/src/Pimple/Container.php:113
#1 Pimple\Container:offsetGet in /Users/Artyom/domains/gravru/system/src/Grav/Common/Grav.php:288
#0 Grav\Common\Grav:process in /Users/Artyom/domains/gravru/index.php:38

How to fix it?

[mblode]

I think in Antmatter, a post.md is actually called blog_item.md. Therefore you just need to change the name of the blog post files in /pages

[Sogl]

I think that's because I use ru default translation. After re-saving all posts in admin panel every post.md became blog.ru.md and finally it works with Antimatter.
But now it doesn't work in Medium, just shows me empty content section.