Here is how I accomplished it with different elements than those that you are using.
Have you inspected the DOM to ensure that you are targeting the correct element? The reason I ask is due to the fact that I glanced over the new docs and did not see a reference for ion-tabs. The following is all that appears in the documentation:
||Background color of the tabbar
||Background color of the focused tabbar
||Border color of the tabbar
||Text color of the tabbar
||Text color of the active tab
Don’t forget about Shadow DOM!