Use the contour filter to display isolines

Add a contour filter

Chose the variable, you want to contour by, use the red x to get rid of the default contour level, use the scale icon (below + and -) to create a number series, (see below), finally select the levels 100 and the 0 and use the + to add -50 and 50 levels below them.

Adjust the line width to 5 by searching for line in the search box in the top and then adjusting it.

Select BrBG as colormap, invert it, and set the value range to -100 to 100.