Actually, to change the transparancy of colors (and colors themselves), you can go in ...\Rainmeter\Skins\TwitchLiveFollowers\
resources and open colorVars.inc in your favorite text editor.
There, just add a fourth number to the desired color to change its transparency.
This number has to be between 0 (full transparency) and 255 (full opacity).
Example with the first one : colorTitleBar=107,0,50,100
Save the file and refresh your skin.
Note that any other number (greater than 255) will make the color fully transparent.
Thanks a lot for this skin again, I'll post what I've done with it later