If you are keeping Archives, categories, Navigation and Tags widgets on buttom widget postition [ Buttom1, Buttom2, Buttom3, AND Buttom4] . Then you can change color setting from custimizer.
For this you need to follow following steps to fix your issue.
1> Go to dashboard-> Apperance-> Customize-> Colours-> there you will find many setting . The following setting are useful to change color of widgets on buttom widgets position
[ Bottom Widgets Title ]: is used to change color of widgets title eg: Archives, categories, Navigation and Tags widgets as in your screen shot
[ Bottom Widget Text ] : is used to change color of text
[ Bottom Widget Link ] : is used to change color of link eg: link inside Archives, categories, Navigation and Tags widgets title.
[ Bottom Widget Link on Hover ] : is used to change color while hovering on link.
IF this procedure helps you then no need to use custom css that is given before, just removed that.
To change color value in custom css , please use this tutorial for reference, so that you change color as per your requirement.