Is there anyway or work around to enjoy the dark mode feature of macOS Mojave without updating to it? I have an old iMac running macOS High Sierra 10.13.4 now it has many default extensions and services disabled to boost up the OS experience. 

Every time I upgrade to a major OS release macOS changes everything to default and installs unnecessary things. These unnecessary things slows down my mac. But I love the new dark mode in macOS Mojave. I want the new dark colored dock with white outline and everything dark included in the latest update.

But, I don't want to fully upgrade to macOS Mojave. So, is there anyway or work around to enjoy the dark mode feature of macOS Mojave without updating to it?
Briefly, no.

Why not install Mojave on a second drive (or just maintain a backup) so you can test things out with the new feature that shipped so you can revert as needed to your old system or data?

The framework and app updates to support this new feature are widely baked into the OS so you would need to have a poor excuse for dark mode by inverting the entire display using accessibility on High Sierra and lower.