Support a second design system

Summary 💡

Provide a second theme the community would love. It has to gain traction on its own to make it a successful effort.

Examples 🌈

One example of a component library that supports multiple themes:

https://seek-oss.github.io/braid-design-system/components/Button

Motivation 🔦

Material Design doesn't have a monopoly on the design systems used by developers and designers. During our last survey, we had 28 feedback to improve custom themes and provide a new theme.

The issues with Material Design:

• Feels a lot like Google. Moving away from this brand can require more overrides than is desirable.
• Community love/hate relationship with Material Design. It's polarising.
• I suspect that making a good looking UI could be made easier, too many shadows.

Proposed solution

We bring on board a designer/developer (somebody with both skills, but has to be a really great designer, no need to have strong development skills) to build a theme using a limited set of constraints:

• Create a new repository under a different name in the mui-org organization.
• Use the unstyled components we are working on #6218. This should save a lot of time in the process. This would create a healthy constraint to make sure that the unstyled components truly deliver (hopefully it will force us to improve them). It would also give the unstyled components credibility.
• Use the tooling infrastructure. Our CI infrastructure is pretty solid at this point. We can make it work across repositories as we do it with material-ui-x.

A few elements that we could consider setting as constraints but we are not fully convinced, it could actually harm:

• Use the same documentation infrastructure
• Use the same styling solution

Related issues

• #18908: Material Design studies x4 themes
• #21911: iOS theme
• #23519: true dark theme
• #21113: document the themes of the community

Benchmark

https://trello.com/c/02I7LL5D/2491-introduce-a-second-theme

Start by reviewing the unstyled components work in #6218 and the tooling setup referenced through material-ui-x. Define the scope and constraints for a separate theme repository, including whether to share documentation and styling infrastructure. Done means a decided, community-supported plan for building and maintaining the second design system.