(Accessibility) Icons in Discover table are invisible on focus

[myasonik]

Steps to reproduce

1. Expand a document on the discover table
2. Tab through the table to reach the filter icons

Actual Result

• Tooltip shows up but the icon does not

Expected Result

• The icon should show up when focus is set on it. Ideally, but not required, when focus is in the row, all icons in the row should become visible.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)