Changing the XFCE panel's opacity.

First right click on the panel you wish to make transparent and select panel > panel preferences.

Once in the panel preferences menu switch the tab to appearance:

WARNING

Don't change the actual opacity slider unless you want the text also being transparent

Go to the background section and change the style to solid colour, go to the select colour section and click custom colour.

Lower the opacity on the custom colour until you get to the level you want and press select then select your new custom colour.