Please put this piece of code into your childtheme style.css

#cp-branding:after{background: rgba(0,0,0,.2);}

0,0,0 = refer to black background color
.2 = refer to the transparent color value, so you can change that value to fit your needs. 
possible value are .1, .2,.3... up to .9, default value there is .6

***don't forget to clear cache after you made changes on your childtheme style.css editor.

Hope this helps