After installing VDA 7.6, the following Local Group Policy was default enabled and set to a default value on VDA server.
If you manually changed the Local Group Policy [Load a specific theme], it will be reset back to NewBasic.theme after VDA restart.
Local Group Policy:
Local Computer Policy -> User Configuration -> Administrative Templates -> Control Panel -> Personalization -> Load a specific theme
Default value:
C:\Windows\Resources\Ease of Access Themes\NewBasic.theme
If you want to change the settings of the Local Group Policy [Load a specific theme], you might need to set it by AD policy. Or you can back up the current NewBasic.theme file, and then rename your own theme file to NewBasic.theme