Hello, Steven Tay,
We apologize, but we are having difficulty understanding your question.
There are five areas in Theme Options > Theme Custom CSS (https://prnt.sc/hykTq7ebcIqW) which related on the particular device and have a hint about the screen size which will be affected by custom code added there. The Global CSS area means that the code will work on all devices.
We hope this information is helpful.
Kind Regards,
8theme team