We followed below package to generate a sass based theme
https://github.com/SnowdogApps/magento2-theme-blank-sass
We used magent2 frontools to generate the css.
It was a success and can see the theme name in admin side theme list.
In same way we tried to create another theme inside vendor folder.
/vendor/mytheme/theme-blank-sass/
Then create necessary files like registration.php, theme.xml and composer.json.
But we cannot getting the theme name in admin side ?
