BetterDiscord isn't an officially endorsed program; and while likely it won't break discord s Terms of Service, which ban modifying chat channels. However the program has been installed over 5.3m times since 2021, and the developers say that they haven't noticed discord take action against many users for modifying their channel. Regardless it does raise some valid questions about whether or not this is a legitimate program. It's hosted on Windows and if you don't have Windows support, this may not be for you.
The best way to determine if BetterDiscord is legitimate is by trying it out yourself. Download the BetterDiscord program, then install it and click "start". You should be able to see a window pop up that installs some files, then a popup will appear letting you know that you are in the middle of the bot network! At this point you are able to browse through all of the themes that are available, and if you need help deciding which is best for your needs you can simply go into the "chatting" section and click on "support".
After running the program you will be able to pick from several different themes. The two biggest themes that are being offered at this time are "Discord" and "GitHub Repo". These two themes offer everything that you need to create your own personal channel, complete with customizable backgrounds and image options, as well as the ability to customize the messaging system to your liking. There is no evidence that BetterDiscord contains malicious code, however this hasn't been verified either.