You are wanting to change the colour of the lines? The css for this is originally in the style.css file around line 294 and the background image is scanlines2.png. If you want the grey one that you see in the top header and footer, rename the scanlines2 to just scanlines.png
Although, if you want something totally different, you will need to create your own transparent lines as a background.
As I usually say, best to use a child theme when you make this change because then any theme updates that happen, your modifications to core theme files won’t be lost. The other option is to install the jetpack plugin, although overkill, it does have a nice feature to edit the style.css just like a child theme would, but the difference is that you don’t need to use a child theme if you are simply modifying styles only.