first: everything within a " " is a given fixed text.

Therefore go for

1. text ( "Current Score: " + x, width -110, 30);

Thus the value of x is printed and x is seen as a variable because it is outside the " ".

second:

1. use background (0);

in draw()
or paint a black rect() under the text and print the text afterwards :

1.   // -------------- 
2.   // Information in the upper screen
3.   fill(0);    // black rectangle = background for information
4.   noStroke();
5.   rect(0, 0, width, 50);
6.   fill(255);  // Text
7.   text ("Mouse: " + mouseX + ", " + mouseY, width-100, 30);
8.   text ("drag mouse-draw shape; q-quit; esc-drop line; Backspace-undo; d-del; r-CODE", 10, 30);
9.   // --------------