In KDevelop IDE already installed some standard dark themes – Breeze Dark, Breeze High Contrast, Obsidian Coast, Wonton Soup, Zion (Reversed). You can use any from them.
To enable/apply already installed in KDevelop IDE dark theme go to: Main menu → Settings → Color theme and select any appropriate.
Also you can change fonts and colors schema for text editor on dark here: Main menu → View → Schema and select any appropriate.