add: backround theme selection - #7

Possible null reference argument for parameter 'micName' in 'void AudioController.MuteMic(string micName, bool mute)'.

Possible null reference argument for parameter 'micName' in 'bool AudioController.IsMicMute(string micName)'.

Possible null reference argument for parameter 'processName' in 'void AudioController.MuteProcess(string processName, bool mute)'.

Possible null reference argument for parameter 'processName' in 'bool AudioController.IsProcessMute(string processName)'.

Non-nullable field '_eventHandler' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the field as nullable.

Non-nullable field '_mainWindow' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the field as nullable.

Converting null literal or possible null value to non-nullable type.

Non-nullable property 'DisplayName' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

Non-nullable field '_mutex' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the field as nullable.