there are multiple ways of doing this.
if you are using a Button node, you can simply set the "normal" look and the "focused" look in the textures property.
another cool thing about control nodes, such as Button, is that you can change the text focus in themes without gdscript.
Select the desired node, go to Theme Overrides in its properties, enable Font Color Focus and set the desired color.
this might be what you are looking for.
otherwise you can use the existing focus_entered() signal and gdscript and connect it to itself.
modulate = Color(2,2,2,2)
modulate = Color(1,1,1,1) #to bring it back to normal
by the way, to identify a node that is currently in focus, you can do
for example if the signal is connected to a parent node: