Who to make editor mobile responsive

hi there, hope you all had a good xmas !

 

Ok i want to make this page with the editor on it more responsive /groups/add/36  there's parts that are 100% and others just go half way and don't fit in the screen.

Where can i find this page to try make it right please ? 

 

 

strangely enough its 100% on the edit profile page 

 

 

 

thank you !