I prefer to use the standard old mIRC theme.
There is no old standard theme. The way old versions of mIRC displayed MDI windows was due to Microsoft not updating MDI windows to use the standard Windows 10 theme because they deprecated the use of MDI windows and stopped supporting them.
If you really want to force mIRC to behave like a Windows 7 application, you can do this by changing the application compatibility of the executable. Right-click on the mirc.exe or the mIRC icon, select properties, and you can then change compatibility mode to Windows 7.
