Hi there, and welcome to the forum!
Agreed, it would be really nice to be able to set defaults that made the charts look better. (Great wording, and couldn't agree more!) Unfortunatley, we don't have a way that I'm aware of to set defaults. I think that would be an awesome add to Excel, but I can't see it happening any time soon.
In the mean time, you could use a little VBA to change all the lines on a chart to a thinner weight.
To use this, press Alt+F11
Find your workbook in the Project Explorer (usually the window on the left)
Right click your workbook and insert a new module
Paste the following in the pane that opens up:
Close the Visual Basic EditorCode:Sub MakeThinLines() Dim lsc As Long With ActiveChart For lsc = 1 To 100 On Error GoTo ExitPoint .SeriesCollection(lsc).Format.Line.Weight = 1.25 Next lsc End With ExitPoint: End Sub
Now, any time you want to reformat a chart in that workbook, Press Alt+f8 and run the "MakeThinLines" macro.
Hope it helps,