I tried putting it in my child theme first without success
You have to be careful when you do this. Two things that could cause a fail here:
1) You copied the entire parent style sheet over to the child theme folder. That’s a no-no. (It just causes all sorts of problems.)
2) You have code after this code that also affects widget links. The rule with CSS is that you can specify a declaration as many times as you like, but only the last one will actually be applied.