How change bg color of panel buttons?

[Dancer18]

I would like to inspect the UI to lighten the background color of the panel buttons for better contrast.
Unfortunately I cannot reach the panel bar with inspect UI. How can I do that to get the right custom css for it?
In this screenshot the mail button should be highlighted but that hasn't contrast enough:

I could solve the issue myself.
I had already in place (custom.css) this code:

#switch .addwebpanel-wrapper > button.active, 
#switch > .button-toolbar.active, 
#switch > * > .button-toolbar.active,
#switch > button.active {
background-color: var(--colorHighlightBgAlpha) }

Instead of --colorHighlightBgAlpha I found a more conveniant item with inspect UI:
--colorFgFadedMost for more contrast. However, I had to add !important; to it:

#switch .addwebpanel-wrapper > button.active, 
#switch > .button-toolbar.active, 
#switch > * > .button-toolbar.active,
#switch > button.active {
background-color: var(--colorFgFadedMost) !important; }

Here is the result: