From LoTROInterface Wiki
Sets the opacity of the window.
This Sets the current opacity of the window which determines its level of transparency when being displayed. Valid values are infinate. Usually 0.0 to 1.0, with 0.0 being fully transparent and 1.0 being fully opaque.Opacity is a special property that is only valid for certain controls. Specifically, Windows will make use of opacity to determine how they are blended with the game.
Some assets still look transparent with a value of 1.0.
Turbine.UI.Lotro.Window requires a setting of ~1.1 to get fully opaque.
Turbine.UI.Lotro.GoldWindow requires a setting of ~1.3 to get fully opaque.
So to be sure of getting fully opaque assets, use a high number. There doesnt seem to be a ceiling/roof for the value used.