Change Mobile Side nav colour

Hello

My website has a black background. When my tablet is in the portrait position, the 3 bars appear for the side navigation to slide out.

The problem is that the 3 bar are also black, so they are not visible. Once the bars are click they turn yellow, but as they are black initially, no one will be able to see the bars to click them.

How can I change the colour of the bars from black to white to make them visible.

Many thanks