Hi! I did see that @ahmaddanial did reply already with information, but as I have actually personally been looking into the same topic for my own use of the platform, I thought you would find this useful as well.
I know you stated that you wanted to avoid any technical work such as code modification, but the most complete way to access the ability to customize and modify your installation of Mattermost to fit your needs best is (in my opinion) using the mattermost web app, which provides you with the full filesystem of mattermost in one bundle, easy to install, and even more configurable than the mattermost server install.
You can find the code on Github here, and the documentation on how to install and deploy the Mattermost Web App is linked in the description of the Github Repository.
I hope you achieve the customization your looking for, if you have any difficulties please feel more than welcome to open another thread!