Add custom css for last element in left sidebar

Hello Team,

I have added css to have a sticky left sidebar where I add related posts but some times there are not enough related posts so the left sidebar is only half filled.

Now the background of the website theme is grey and the left sidebar background is white so it looks quite weird (sample post shared)

It is possible some way to have the complete sidebar have white background even when there are not enough related posts? May be add a margin or max-height to last element of left sidebar which fills up the empty space? or some other proper way to achieve this.

Of course I can't anticipate the no of related posts which each post will get but a generic way to fill the entire available space in the left sidebar with white background.

Thanks in advance