Update themes.md add theme

[A5yncX]

Summary

Add darcula theme https://github.com/A5yncX/docsify-theme-darcula

What kind of change does this PR introduce?

Docs

Does this PR introduce a breaking change?

No

Tested in the following browsers:

• [x] Chrome
• [x] Firefox
• [x] Safari
• [x] Edge

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Name	Status	Preview	Comments	Updated (UTC)
docsify-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 11, 2023 11:31am

[jhildenbiddle]

Hi @A5yncX --

Please add this theme to the awesome-docsify repo under the Themes section. Thanks!