I applied following css:
.v-panel {background: yellow;}
this colors the panel header and a small border area in yellow. But in the centre there is still white portion i can see.
So how to color that as well?
Depending on the browser, you may need to override additional components. This is true for Chrome/Safari in particular. I had to use the “inspect element” right-click feature – look at the right hand side and play with the checkboxes to turn specific rules on and off.