Hello Faruk,
I am sorry for the delay of this reply as it weekends for all of us here.
is the a way of changing background color
Do you mean you want to change the background color of the composer editor? If that so unfortunately it is not possible with the current system.
how can user switch editors
Only admin can change the editor of the user. Administrator can change the editor from backend > easyblog > authors > select users > Editor,
http://screen.stackideas.com/2015-06-22_1030.png .
Hope these help.