Succeeded in fixing this by using the background property at the
/* Alternate shading for lines */
li.L1,li.L3,li.L5,li.L7,li.L9 { }
and also using the !important
keyword to overwrite the defaults.
So this is how it looks like:
/* Alternate shading for lines */
li.L1,li.L3,li.L5,li.L7,li.L9 { background: #color !important }