Text-Editor - Customizing?

inside of tasks or issues sometimes i have a list of things to do.
To mark those things which are done, i change the background-colour, for example green - now i have a good overviev.

But if i print the issue, there are no colors on my paper (yes it is a color-printer)

The solution - and clearly a better way, would be to have the button “strikethrough” (a horizontal line over the text) inside the editor.
Many editors, like in wordpress or joomla, do have this button in the editor.

My question: how to customize the editor? How to add this button?
I did not found a way to customize

thanks and best regards.