[Theme] Support additional custom themes

[Lagicrus]

• I have searched the issues of this repository and believe that this is not a duplicate.

Summary 💡

The ability to have more palette types.
Support tertiary colour options.
Whilst light & dark is extremely useful, I want to add an addition theme for Colour Impaired viewers. Which to my knowledge is not supported as a toggle for palette/type.
For another use case, having other themes that a user wants to use besides just dark/light.

Examples 🌈

https://www.ibm.com/design/v1/language/resources/color-library/ Contains some examples at the end for colour impaired safe colours
https://www.w3.org/TR/AERT/#color-contrast Contains a recommended formula for the colour difference to help readability for people with impaired vision

Motivation 🔦

I am hoping to make it easier for developers to add more theme options to help support people who either want a different theme outside of Dark/Light, and those of us who have a vision impairment were having a different colour scheme designed to help would be of great use.

[oliviertassinari]

@Lagicrus I couldn't agree more. I think that it would be awesome to include these themes too https://material.io/design/material-studies/about-our-material-studies.html.

[eps1lon]

@Lagicrus I couldn't agree more. I think that it would be awesome to include these themes too material.io/design/material-studies/about-our-material-studies.html.

These are covered by #18908

@Lagicrus I think from the description of the issue this is more about custom colors not completely different themes? If so then I think #13875 already covers this?

[oliviertassinari]

@Lagicrus Maybe you are looking for a high contrast mode? It's somewhat related to #13174.

[Lagicrus]

@Lagicrus I couldn't agree more. I think that it would be awesome to include these themes too material.io/design/material-studies/about-our-material-studies.html.

These are covered by #18908

@Lagicrus I think from the description of the issue this is more about custom colours not completely different themes? If so then I think #13875 already covers this?

Thanks for the links! They seem to be closer to what I was looking for.
Do you think it would be worth linking the IBM link to #18908 as an additional example?

I will admit that I seem to have drifted from the original scope of the request from an additional set of colours (on top of Primary/Secondary) to a whole new theme itself so #13875 is of interest to me as well, so thank you for that one :)

I see this as closed as it seems to be better covered by two preexisting issues, can't wait to see what happens with them.
Thank you @oliviertassinari and @eps1lon 👍