Here is an example of a plot with the "Detailed" PlotTheme in version 10:

plot = Plot[{x^2 Sin[x], x^2}, {x, 0, 13 \[Pi]}, PlotTheme -> "Detailed"]

FYI, it always generates a legend (even if there is only one curve).

But you can see the options that have been set for this plot (but which does not include the legending) using:

Rest[List @@ (First@plot)]

Taking the results of this as a guide one can begin to replicate the styling using

Plot[{x^2 Sin[x], x^2}, {x, 0, 13 \[Pi]}, 
 PlotLegends -> "Expressions", Frame -> True, Axes -> False, 
 GridLines -> Automatic, 
 GridLinesStyle -> 
  Directive[GrayLevel[0.4, 0.5], AbsoluteThickness[1], 
   AbsoluteDashing[{1, 2}]], 
 BaseStyle -> {FontFamily -> "Helvetica"},
 FrameStyle -> Gray]

So, comparing the PlotTheme -> "Detailed" in version 10:

to the version 9 result using the options above: