Fix @shade_color on Zorin OS 16 palettes

[dominichayesferen]

Initially I didn't realise where this colour was used, but now I know I reverted it back to Adwaita's values, which appears to be very similar to how shadows look in Zorin OS 16

Checklist

Before submitting the PR, I checked:

• [x] I've only modified one preset
• [ ] I've added the URL to my raw preset in presets.json
• [x] I've checked the format of each json files I modified
• [ ] (optional) I've added screenshots
• [x] The preset name is following preset naming guidelines.

Publication

• [x] I want to add my preset in the preset list.

Thanks for your submission we will review and merge it as quick as possible.

[daudix]

Thank you very much!