Ubuntu(Mini) - How to replace the shell of system message dialogs with my own version?

I'm using **Linux Ubuntu 14.04 LTS** installed on **VirtualBox** using mini .iso and trying to make my own distribution based on it. I know, support of that version ends in 3-4 months, I'll update it on my new computer. Of course, I'm developing desktop environment, taskbar, etc. But I don't know how to customize the **message dialogs** (error, warning, question and information) of it. Now I want to replace the shell of these dialogs with my own version. I'm using "**Openbox**" as base for desktop environment. And **x11** and **GTK** is installed on it. Are these message dialogs related to GTK or X11? *(Note : This is my first question in StackExchange)*