How to configure custom themes in magento 2.

1.) First create theme.xml as per given below

app\design\frontend\ThemeName\ThemeName\theme.xml

Magento/luma

2.) Then create registration.php file as per given below

app\design\frontend\ThemeName\ThemeName\registration.php

\Magento\Framework\Component\ComponentRegistrar::register(

\Magento\Framework\Component\ComponentRegistrar::THEME,

'frontend/ThemeName/ThemeName',

__DIR__

);

3.)Then configure it from backend content -> design -> configuration.

4.)After then click on the edit link in the given grid view.

5.) Then in the applied theme drop down list select your theme and save it and then flush the cache.