fix: move theme style into Docus plugin

[farnabaz]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Originally reported in #92 (comment)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

[vercel]

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🔍 Inspect: https://vercel.com/nuxtlabs/docus/3cSSfRJykqE9ABcc7Yh7AxpDX8w2
✅ Preview: https://docus-git-add-theme-style-nuxtlabs.vercel.app

[atinux]

When navigating on client-side, the colors disapear: https://docus-m1ty2lovu-nuxtlabs.vercel.app/

Also, can we be sure that app.head.style is an array?