I want the user interface a little bit darker, like most of other software these days, kind of a gray or dark gray… The white color is effecting my vision and thinking. If you really consider my feature request please reply.
I just asked about this feature the other day actually. I agree and hope to see it added sometime. 
TBH, implementing skins on GDevelop might be a better thing because people have different preferences on how their UI should look like. Some like light UI, some like dark UI, some like neutral UI.
My suggestion was to add a dark skin. Not replace the existing look.
The problem is that, except the color the event editor and of the Ribbon (the Ribbon appearance is already customizable in GDevelop options), GDevelop can’t change the color of the other widgets (button, …) as they are rendered by the OS itself.
Alright good to know. I got gdevelop looking ideal for my needs now.
Change your PC settings to use one of the dark themes then in options change the colors to whatever you like.
Mine looks like this.