The only adjustment available for layer symbology is transparency, which is located in the Layer window.
- Select the layers tool.
- In the properties window that appears, select the "..." to the right of the layer you want to change.
- In the properties select Transparency. In the transparency adjustment area, adjust the transparency. Dragging the transparency slider to the left will increase the opacity of the layer, thus increasing the visibility of the layer, to the right will increase the transparency.