Did you try to install/upgrade Magento 2.2.4 version? We sure that you also met the “Something went wrong while saving this configuration: Area is already set” error message in you back-end while trying to change the default theme. The problem came from the core of Magento.
Luckily, the solution to fix this is quite easy. But please note that this is Magento 2.2.4 know issue and below fix is just a temporary fix, you should not directly change Magento core file (override or create a plugin).
You can apply “How To Fix Theme Switching Error Message In 2.2.4” to fix it now and remove it after Magento developers fix the core issue.