The main fill color with silica is transparent

As the main fill color might now be actually used wih the Silica
theme wen running with the Silica UC backend, set the main fill
color to be transparent in the Silica theme.

themes/silica/theme.conf

@@ -26,7 +26,7 @@ name="Silica theme"
 ##   the numeric alpha specification is ignored
 
 [colors]
-main_fill="#1a1a1aaf",1.0
+main_fill="transparent",1.0
 main_outline="red",1.0
 main_text="red",1.0
 widget_background="black",0.65