1. Download the zip from here
2. Goto your MultiMC installtion path
3. Extract theme.json and ThemeStyle.css to [INSTALLTION_PATH] \MultiMC\themes\custom
4. Goto MultiMC -> settings -> launcher -> user interface
5. Click and expand Colors
6. Choose the one that says "Blueish grey"
7. Close the window and it should be working

Screenshots: