Which widget are you using? You’ll get drop-down option to select category in those widget where you need to list posts. Also, go to Appearance > Customize > Design Options > Skin Color to enable dark theme.
Glad to know it helped you. 🙂 If you get any issue later, feel free to ask again! I hope you liked our theme, if so, you can help us by providing a review us here: https://wordpress.org/support/theme/colormag/reviews/?filter=5