For all those who want to dive deeper into the FSE (Full Site Editing), the website fullsiteediting.com is a great starting point. The site is run by Carolina Nymark, one of the prominent members of the WordPress community and an excellent developer.
There is a block themes generator on this site that allows you to generate starter block themes in just few clicks.
Now, this generator has been upgraded and it has optional theme.json settings where you can choose colors and content widths.
Check it out here for more details: https://fullsiteediting.com/block-theme-generator/