Native Hungarian theme
The theme uses static resource (i.e. stylesheets and scripts) with custom inclusions from Resources.cshtml. This template is included by Pretty Good Bootstrap Base Theme, the base theme what this theme uses. The usage of PGBBT is not mandatory for DotNest themes of course but it enables to build Bootstrap-based themes quickly - and those themes can be compatible with DotNest, as you can see.
Note that using a base theme only allows you to include static resources from the base theme, Media Theme themes won't be able to fully load a base theme (with templates and everything). So you can use a base theme like demonstrated here, e.g. to import LESS files from it (but the actually used, compiled CSS output is fully contained in this theme). If the theme you use is also available on DotNest (like PGBBT is) you can also directly include resources from it (e.g. load CSS or JS files from it from templates by using URLs).