I don't think you can do that. You cannot fill the colors between those lines.
You can only change the colors of the line.
I'm not sure and if there is any Qlik Sense expert who has already done this can share his/her knowledge.
In qlikview, you can increase the weight of the line but I believe it won't go beyond 10pts. It will increase the width of the line.
Attaching a workaround to your query. Not the cleanest solution; but works!
Things to Note:
Fixate the max values on the charts >> See Axes Tab and Check Static Max and should be taken as the max value in your chart , which in this case is for Dim2 : f Dim3: x
These are two charts placed on top of each other
threshold_Highlighted.qvw 189.5 K