(This is a valid parity issue as the last explicit change made to this feature was in 1.15's release, which fits within the Buzzy Bees and later window.)
When highlighting a button in Java Edition, a bright border is applied around said button, but nothing else. For inventory slots, the slot becomes a lighter shade.
In Bedrock Edition, however, this behaviour is different - both cases will, alongside an outline (including inventory slots), experience an intense visual change with the entire button/slot becoming a bright shade of green. This change is far more noticeable and better for accessibility as a result, and should probably be ported over to Java Edition.
A list of Java Edition UI texture files requiring updating is forthcoming.