Increasing the border thinkness of selected cells when pasting to multiple cells

I am trying copy the contents of the clipboard to the selected cells. The 1st cell I selected has a think border indicating it is selected however the other selected cells have a very thin border. How could I change the functionality so that all the selected cells have a think border?

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@greggkatz,
You may share your runnable simplified console application which can be used to reproduce the issue here along with the template excel file, program output, expected output created by MS Excel and environment details. We will analyze the information and provide assistance accordingly.