docs: add theme variable link to theme docs

[AntonBorovoi]

Please read and mark the following check list before creating a pull request:

• [x] I read and followed the CONTRIBUTING.md guide.
• [x] I read and followed the New Feature Checklist guide.

Short description of what this resolves:

closes https://github.com/akveo/react-native-ui-kitten/issues/1218

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[github-actions[bot]]

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