add core themes (#13021)
* - add evergreen theme
- add more vars for customisation
- css tweaks
* add changeset
* neon fix
* neon theme
* add other themes
* add changeset
* format
* format
* format
* format
* remove noir
* remove evergreen
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Co-authored-by: gradio-pr-bot <gradio-pr-bot@users.noreply.github.com>
Co-authored-by: Freddy Boulton <41651716+freddyaboulton@users.noreply.github.com>
