If this affects something, then I'm use the "Windows 8.1" system and "mIRC v7.63". Changing parameters in: "Options/Display/Options/Styles/" does not resolve the issue of visible border around chat windows.
For clarity, I'll set the color to red. The screenshot shows that the separator strip between the chat window and the text input field is highlighted in red, and blue arrows indicate borders that are visible and cannot be removed or changed in any way.

[Linked Image from i.ibb.co]

To reproduce this problem, you need to style the mIRC client in a dark style, to do this, follow these steps:

   1. On the client menu uncheck several options to disable them "Viev/TreeBar" and "Viev/MenuBar".
   2. Right-click on the SwitchBar (List of open windows), select from the menu "Position/Left".
   3. Right-click on the SwitchBar (List of open windows), select from the menu "Lock Bars".
   4. Download this image to the root folder of the client's mIRC: image for background (or use any of your images in dark colors).
   5. Right-click on the ToolBar, select from the menu "Background/Select" and install the downloaded image.
   6. Right-click on the SwitchBar (List of open windows), select from the menu "Background/Select" and install the downloaded image.
   7. In the ToolBar menu, click on the "Сolors" button and set the color to black in the EditBox and in ListBox.
   8. Right-click by status window in the windows list SwitchBar, select from the drop-down menu "Background/Select" and install the downloaded image.
   9. Then set in the same window menu how to display this image "Background/Tile". (Same for channel and private windows).

Congratulations! You have now set up your client's dark style. Now all the light gray frames and borders of the windows are visible, as in the screenshot above, indicated by the blue arrows.

I would very much like this visual flaw, which spoils all the beauty of your style, could somehow be veiled in your colors in RGB or removed altogether, through the client's options.
I think that many developers of their design for the mIRC client, have repeatedly encountered this drawback and will be grateful if there is such an opportunity to fix it through.

Ideally, it's nice to have something like this in the options, with the ability to choose your own color from the RGB color range, including the ability to completely remove all frames and borders during installation:
"No Frame" and "Border size = 0", so that it was possible to create your own client design, for example this:

[Linked Image from i.ibb.co]

Well, and accordingly, for quick setup and change of style through scripts, add 2 commands: "/frame <on|off> [1-5] [rgb-color]" and "/border [0-5] [rgb-color]"