ERModernLook CSS simple question

Being completelly dumb with CSS, I cannot find how to enlarge the property name labels area in normal edit pages (ERModernLook).
I can see that the width of the label area is controlled by

.Line .LabelWrapper {
    width: 167px;
}

But if I change the width, the background does not change, and I can't understand how to make it fit the label wrapper width without changing the label_bak.png image itself.

I am sure there is an easy solution to this, but I never really used CSS before, and thus cannot figure it out...

TIA

JPM

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