When using the tree-view side menu and you collapse it

When using the tree-view side menu and you collapse it to the right of the screen there’s a huge white space. The light gray doesn’t extend all the way to the right of the screen. How can I fix this or at least change that color to white in the theme? Where is that to change? Thanks.

Hello,

Could you display some image about this problem?

Hello,

Can you describe some step by step to simulate the problem?
I managed to simulate, but in a rather unusual situation. Occurred when, after the page loaded, I maximized the browser window.

I have a tree-menu, a grid application (grid) that is called from the tree menu. The grid application has 9 columns, one of them is a multi-line text field with a field width of 400. With the tree-menu open I have to scroll horizontally to see my entire grid. When I collapse (hide) the tree-menu is when I see the white space as shown above.