Sensible-WP theme, how to change menu-toggle breakpoint from 900-767px?

I’m a 1st time WordPress user and know a little CSS. I have WordPress 4.4.2 and the Sensible WP v1.0.34 theme. I created a child theme which includes some custom css. The parent’s theme’s breakpoint to toggle/display the small menu is 900px, as shown here:

@media screen and (max-width:900px) {
	.menu-toggle {display:block;}
	.menu-toggle {
		background:none; border:none; padding:1em 1.5em 1em 0;}
	.main-navigation ul {display:none;}
}

The parent theme also includes 2 other @media screen sections:
@media handheld, only screen and (max-width: 769px)

and
`@media handheld, only screen and (max-width: 767px) {
.grid {
width:100%;
min-width:0;
padding-left:20px; /* grid-space to left */
padding-right:10px; /* grid-space to right: (grid-space-left – column-space) e.g. 20px-10px=10px */
}’

I want to reduce the theme’s 900px breakpoint to 769px because I have a narrow navigation menu. Note that my target 769px breakpoint is also a breakpoint in the parent theme (see above). But if I only change “900px” to “769px” in my custom css file, this does not decrease the small menu toggle breakpoint to 769px. (In addition if I remove the entire @media handheld, only screen and (max-width: 767px) section shown immediately above, the breakpoint still does not decrease.

Because my website is still in ‘Beta’ and private, I cannot provide its URL. But my child theme’s custom css only includes superficial changes like font-size and colors. The rest of my custom css works fine.

What do I need to do reduce the breakpoint to 769px? Please be specific. Thank you very much.