Add Monokai Pro macOS CSS-only theme

[jesse23]

Theme

Add a CSS-only Monokai Pro macOS-inspired dark theme: #0a0a0a sidebar, #1f2021 content, neutral text, cyan settings links, and pink accent. It includes only plugin.css, its signed public manifest.json, and a minimal landing page. No JavaScript or vault data is included.

Gallery integration needed

Please review and add theme-monokai-pro-macos to docs/plugins.json with URL https://plugins.keeweb.info/plugins/theme-monokai-pro-macos, then regenerate/sign the gallery index using the KeeWeb maintainer signing key. I deliberately did not modify docs/plugins.json because its existing signature cannot be recreated by a contributor. Merely merging the three files without the signed index will not allow KeeWeb to trust this plugin on reload.

Verification

• The CSS signature verifies with the public key in the manifest (RSA-SHA256).
• CSS selectors are scoped to .th-monokai-pro-macos; no remote assets or plugin JavaScript.
• The contribution is limited to these three public theme files. The private setup repository and private signing key are not part of this PR.