Hi Minakshi,
Your observation about changing colour of data series is right. However, you can use the following snippet in the screenshot below for doing the same (this will take care of changing legend colour as well):
In a similar way you can change the width of the data series in the Line chart, but the new width gets applied to all the lines/ data series in the chart.
c.getChartObject().getUI5ChartElement().getPlotArea().setWidth(7)
//setWidth(width) - Set the width of all the lines, ranging from '1' to '7'. If you enter a value outside that range, the line width defaults to '2'.
The code that I had shared earlier using jQuery and CSS for changing width and colour will work only after chart is rendered in DOM for e.g on click of button (i.e., It will not work within update/first update event handler)
Hope this helps!
Regards,
Ria