There is no way to turn it off. The MDI area/windows have been updated to use the standard Windows 10 theme.
The reason for this change was that users have often complained about mIRC looking like a Windows 7 application, due to Microsoft deprecating support for MDI windows and not updating theme support for them.
While I appreciate that interface changes can be difficult for some users to accept, in this case it just wouldn't make sense to revert it back to the Windows 7 theme, or to even add that as an option, now that it looks like a modern Windows 10 application. This is how it would have looked years ago, when Windows 10 was first released, if Microsoft had properly updated MDI support.
For those unhappy with the border width - there has been no change to it. It is the same size as previous versions. The border width cannot be a single line like with desktop windows, it has to have some thickness to allow you to size it and for it to be displayed correctly within the MDI area. MDI windows are using the standard sizing border thickness used by all windows.
For those unhappy with the brightness of the border - it is the same color as all of the other window borders in Windows 10. If it is too bright, you can change the color in the Windows 10 Options/Personalization/Color section.
For those unhappy with the brightness of the MDI background - it had to be changed to match the standard theme. You can change it by choosing an image for the MDI background. Right-click on the MDI area and select "Background" from the popup menu and choose an image. I may add an option that allows you to set a custom color instead of an image to make changing this easier in a future version.
For those unhappy with the line that separates the editbox from the display area in a window - it is now using a standard interface color. However, I am aware that users often use darker themes in the colors dialog and that this clashes with them. I will be adding am option to the colors dialog that allows you to change this color per theme in a future version.
For those who want mIRC to continue to use a Windows 7 theme for MDI windows in WIndows 10, you can right-click on the mIRC icon, select Properties, and then set the "Run this program in compatibility mode" to Windows 7.