Sum cells based on background color

Yesterday I found this UDF designed to sum only the cell that match a specific background color.

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For some reason it's rounding the result instead of giving a true sum. I assume it has something to do with the line "Dim ColIndex As Integer" but I am clueless about how to read this code. Other than the rounding it works great, can someone please tell me what to change so that it provides the real sum and not a rounded integer?

Change Dim cSum As Long to: Dim cSum As Double. LONG is for whole numbers (integers) DOUBLE is for decimals (0.0).

Also, you don't need worksheetfunction...
just:
cSum = cSum + cl.value