By graphs, do you mean the expressions? Not sure if this is what you mean, but press ctrl+shift+i, and go onto console. The run this:

for (let i = 1; i < Calc.getExpressions().length+1; i++) {
  Calc.setExpression({id:i,color:'#000000'});
}

It will change all expressions to black, but you can change color value to a different hex code However, this causes some errors on some expressions, (e.g. this that don't have colors) and isn't foolproof.