Hi,
i’am adding a custom inline editor. But i didn’t found how to add custom css class at it.
thanks 
Hello, @jabo!
I offer you to watch a detailed tutorial (https://www.youtube.com/watch?v=0rIPrC0GtME) about how to implement inline editing.
There you can find an explanation of using Custom inline editor (1:05). In this example, the developer specified the colors of tasks. He explains every step of implementing a color picker.
First of all, you should use the code from the documentation (https://docs.dhtmlx.com/gantt/desktop__inline_editing.html#typesofeditors) and use it as a template. Then the functions should be changed in a necessary way (there is a good instruction on how to do it in the video above). After that, you should add a new column to the grid and attach the editor to it with the help of grid config. The next step will be display colors inside of the task grid (where you can specify the background color of the tasks).
And finally, if you don’t like an HTML5 color picker, you have an opportunity to change it. There is also a good example of using jQuery plugin in the tutorial (3:38).
The code using in the video: https://next.plnkr.co/edit/tB8sECIYfsGs8bZZ?preview
(or you can also find the links in the video below)
Does it help you to solve your issue with a custom css class?