Change the separator line format between mails in Outlook

In Microsoft Outlook, in this case 2016, but also 2013, possibly older.

I need to change the color of the auto-inserted line, when replying to mails.

I know it can be done, since I have a customer that uses differently colored lines by default when replying or forwarding mails. I tried asking, they have no idea how it's done :-/

Here are some pictures to illustrate what I need changed:

Note the two horizontal lines, one grey, one blue. The grey is mine, with the default #E1E1E1 color.

The blue one is one my client forwarded, the one I'm aiming to replicate. (#B5C4DF)

I found out where the line is applied. When placing the cursor in the "From" text of the message header, and choosing to format Borders and Shading..., it can be modified manually.

Here the settings can be seen. But only manually in this case.

I need to know how to edit the default values.

Is it an Outlook setting? Registry maybe? Or is it somewhere in NormalEmail.dotm perhaps?

I'm at a loss, and extensive web searches have revealed nothing :-/

You can modify horizontal lines by double-clicking them :)