Gruvbox Theme for Unreal Engine

I've re-created the gruvbox theme for the Unreal Engine. The colors are from https://github.com/morhetz/gruvbox. Feel free to offer suggestions.

Install instructions:

1. Install Gruvbox-UnrealEngine.json file, or clone the repo.
2. Navigate to your Unreal Engine 5 installation folder.
3. Navigate to Engine/Content/Slate/Themes. (Create this directory if you don't have it.)
4. Copy the Gruvbox-UnrealEngine.json file to the Themes folder.
5. Open the editor preferences window under Edit > Editor Preferences.
6. Under the Active Theme dropdown, select gruvbox.

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