Use correct fallback color for decoration_border.
Use correct fallback color for decoration_border.

The newly introduced decoration_border color incorrectly uses the default value
for "background" as a fallback when not specified. Instead, it should use the
user-specified "background" as a fallback.

fixes #2149

Airblader/i3-original,stapelberg/i3,cornerman/i3,strake/i3,stapelberg/i3,MForster/i3,i3/i3,EvilPudding/i3,Airblader/i3-original,i3/i3,Airblader/i3-original,avrelaun/i3,Airblader/i3,Chr1stoph/i3,Airblader/i3,shdown/i3,Matmusia/i3,Matmusia/i3,avrelaun/i3,strake/i3,cornerman/i3,MForster/i3,shdown/i3,EvilPudding/i3,strake/i3,acrisci/i3,MForster/i3,i3/i3,Chr1stoph/i3,MForster/i3,stapelberg/i3,Matmusia/i3,Chr1stoph/i3,acrisci/i3,Airblader/i3,shdown/i3,cornerman/i3,acrisci/i3,stapelberg/i3,Airblader/i3-original,acrisci/i3,Airblader/i3,acrisci/i3,cornerman/i3,EvilPudding/i3,i3/i3,shdown/i3,Matmusia/i3,avrelaun/i3,strake/i3,avrelaun/i3,EvilPudding/i3,Airblader/i3-original,strake/i3,Chr1stoph/i3