To customize KISSCMS according to your needs you'll need reference material to guide you. The best place to find information is to read the official Wiki on Github
The main areas a web developer might be interested creating custom code for are:
Obviously the easiest way to modify KISSCMS is to apply a different front-end skin. All related files are in the "html/" folder. The assets folder contains static files (images, css etc.) for the website layout and the templates folder contains the different layouts of the website.
For more information on how templates work please refer to the wiki docs: https://github.com/makesites/kisscms/wiki/Guide:-Templates
You can also find a number of officially supported themes (that is themes that are tested to be compatible with the latest release) at this Github repo: https://github.com/makesites/kisscms-themes
These are content parts of a webpage, usually backed by a class that defines the required dataset and the view used for display.
Here is more information on how to work with sections: https://github.com/makesites/kisscms/wiki/Guide:-Sections
Please note that if you want to change the existing sections it is advisable that you create a BASE folder for your website. read more
If modifying sections is not enough and you need full fledged custom work done, that could include new controllers and models, it is better if you bundle your work in a plugin. That way you can easily separate your work from the distribution files (which mean painless upgrading) and also ensures that your code is re-usable in other similar projects.
This guide will get you up and running for creating your own plugins: https://github.com/makesites/kisscms/wiki/Guide:-Plugins