Hi,
To change the image there are graphical and console utilities (cl-builder). You can modify the ISO from the installed CL, or by booting from a flash drive. There is a video, unfortunately in Russian, in it you can understand the sequence of actions. Firefox is installed on the video. During the preparation of the video in the distribution was Chromium
You can change topics according to this guide. In the /etc/calculate/ini.env file, you can also set other interesting settings that will be used when creating a new user session.