Is it possible to use blank or luma theme for mobile and custom theme for desktop in magento2.
You can achieve this by design configuration from magento 2 admin.
Go to STORES > Configuration > General > Design -> User-Agent Exceptions
Search String -> Search strings are either normal strings or regular exceptions (PCRE).
For example : iPhone|iPod|BlackBerry|Palm|Googlebot-Mobile|Mobile|mobile|mobi|Windows Mobile|Safari Mobile|Android|Opera Mini
Design Theme : Select the theme you want to load.
You can also use screen resolution regular expressions for loading theme based on resolutions.