Hello @alexVillalobos
Greetings of the day..!!
There is no other way to include this because if you make any kind of changes and save the file it doesn't get compiled npm run dev helps to compile all those changes so after the changes are reflected you don't need to run npm run build again because the build is already present in it.
So overall If you run the npm run build again you have to run npm run dev to compile all the changes.
Kindly check our custom theme blog and video tutorial here.
https://bagisto.com/en/create-custom-theme-in-bagisto/
https://youtu.be/K3FdBDsXAH8?si=eXaG8kyrdLs1zeTM
Thanks & Regards..!!