Not every theme can have its color edited.
To find out right click on the desktop, a window will open and then click on Change Desktop Background, choose the Theme tab. Your theme will be highlighted. Click on Customize, then in the new window, choose the Color tab. If the color selections are greyed out, you can not change the colors in that theme. Otherwise, customize away!
Malachi 3:10 NRSV
...says the LORD of hosts... I will not open...windows...for you...
Linux user since 2001 pda; 2006 lappy