The transset Linux command manual page and help. transset is a simple program for X servers that supports XFIXES, DAMAGE and COMPOSITE extensions. Allows the user to set the transparency of windows. 
The transparency value is a number between 0 and 1, where zero is completely transparent and 1 is completely opaque. If no opacity argument is specified, the default value of 0,75 is used for the opacity value. If neither --inc nor --dec is specified, opacity is treated as an absolute value and set to the given opacity value. If --inc or --dec is specified, opacity is treated as a relative value to increase or decrease the current opacity.