how to remove white lines in twenty thirteen child theme

Try using Firefox with the Firebug add-on for this kind of CSS troubleshooting. Or use whatever developer tool is available in your web browser.

Although I do not usually use Firefox, it seems that Firebug works like Chrome’s “Inspect Element” tool, and I have not been able to use this tool to find the part of the code that is creating the lines in my page.

or the one that horizontally splits my foote

https://snag.gy/hpO7Q.jpg

the vertical white lines that flank my header

What vertical white lines?

Firebug is far more powerful than Chrome’s Inspection tool. apart from anything else, Firebug allow you to change and add to CSS temporarily & on the fly.

In all three browsers I have tested, I see one vertical white line to the left of my header, along the header-map, navbar, and slideshow (front page only), and on the right the line only appears next to the navbar and slideshow (front page only).

Can you please explain the meaning of the link you provided?

Can you please explain the meaning of the link you provided?

Do you not see the CSS responsible for the line?

Here are some tips for how to use Chrome’s developer tools
https://dailypost.wordpress.com/2013/07/25/css-selectors/

https://dailypost.wordpress.com/2013/06/21/css-intro/

https://dailypost.wordpress.com/2013/08/29/css-matched-rule-pane/

In all three browsers I have tested, I see one vertical white line to the left of my header

https://snag.gy/avqlh.jpg

Andrew, I can tell that you are highlighting a line of code, but I do not understand how this is supposed to help me fix it.

Sorry I assumed you had experience with this, what you need to do is look at the CSS (on the right column). You can see for example on your header issue that there is a left and right border being applied to the element that has the class of “site” (.site).
To override that style go to your Child Theme style.css file and add:

.site {
	border: 0;
}

Thank you! That’s what I needed.