Sorry for not providing more information, But I managed to find the solution. I'm trying to make a Blender theme close to Adobe PS, just grey tone colours. But what I was after is located in the: User Interface> Options. ibb.co/XCLY22f

I'm in EEVEE, but I also checked Cycles. I didn't know that depending on the render engine the theme colour can change. It's somewhere in the Themes options which I cannot locate.

First time I used Smooth Modifier, fixed the shading issue, thanks for that!

Maybe those panels have opacity settings to make the grid appear behind them.

Thanks for your reply Josh. I should be more specific. Blender's default panels are on the left and right, but have the grid expanding to the boundary of the viewport. The theme I have downloaded, has the grid cut off like a square shape. ibb.co/fn4HZKQ

So this line is a Gizmo, thanks for the link.

Its the loop cut line.