Comice OS is inspired by Mac OS so the author was very careful to pick a nice theme and icons to satisfy users who love the desktop eyecandy of Mac OS, the default gtk theme of Comice OS is Adwaita Cupertino, the default icon set is Faenza Mac. But Comice OS also comes with many other nice themes preinstalled, if you want to change the GTK theme and icons in Comice OS, here is how to do it.
First, if you need to use extra themes, you can download the themes and extract the themes to /usr/share/themes or ~/.themes. For icon sets, after downloading the extra icon sets, you need to extract them to /usr/share/icons or ~/.icons ( you also need to create the .themes and .icons folder first)
Next, click on the Comice OS button on the right of the top panel, search for "advanced settings" in the search box and you will find this option.
Click on the advanced settings button and a window will appear. And on the left side of the advanced settings window, you will find the theme section, check it and you will have a menu to edit theme, cursor, icons, window theme ... just like the image below