feat: add fysetc theme

[zzcatvs]

Description

This PR adds a Fysetc theme.

Related Tickets & Documents

none

Mobile & Desktop Screenshots/Recordings

[optional] Are there any post-deployment tasks we need to perform?

none

[meteyou]

Thank you very much for your PR! We appreciate your contribution, but we want to clarify that built-in themes are limited to our vendor sponsors or open-source projects (such as Voron and VZBot). Fysetc is not currently a vendor sponsor of Mainsail, nor is it an open-source community. Built-in themes should give our sponsors a little bonus and recognize their support.

However, you are welcome to add your theme to the community themes. This works via our docs repo. Create a repo with the theme data and enter the theme here via a PR: https://github.com/mainsail-crew/gb-docs/blob/main/_data/themes.csv. This will add this theme to our docs page: https://docs.mainsail.xyz/overview/features/themes/themes

We hope for your understanding and look forward to more contributions from you.