Thanks for the response.
I tried 'compact' and that was a bit jaring and never seen this view before, reverted back to normal.
Tried out the option to make channels to open as desktop windows, this gets things looking good but the side of effect of them being their own floating window and not contained within mirc is not ideal for me with how many channels I am apart of.
I had a copy of mirc 7.66 running on my laptop and loading that up I can confirm that using 'normal' and MDI windows there was no thick borders.
This is how 7.67 also was for me.. until I clicked on the cascade button and since then I've been stuck with the thick borders and cant undo it.