Twenty Sixteen photo gallery not centered ONLY in medium size "tablet" view

Hello,

Gallery on “portfolio” page anchored to the left of center when I shrink the window to appoximately 3/5 of my screen width. Coincidentally does this when using the “tablet view’ of the web page from WP admin “customize” console as well. In full screen or mobile mode and minor adjustments of window width, gallery centers perfectly. I wanted to include a screenshot but I cant so if you go to my page, click the portfolio page, then shrink the browser window to about 5:3 aspect ratio you’ll see the pictures jump to the left side of the browser window.

Password to access site: showmethemoney

I’ve tried a number of center text, margin auto and other things I could find in the forums addressed to .gallery in my child theme style.css but nothing works. I am really surprised this theme works this way out of the box, since it centers the gallery in mobile view and full screen.

Im using very vanilla Twenty Sixteen Theme, Google Chrome current version on a Windows laptop with 1920×1080 screen. Problem is not changed by disabling plugins or deleting my child theme values. But here they are nonetheless:

‘/*
Theme Name: twentysixteenchild
Theme Uri: https://noflashnozoom.com
Author: Joshua Rose
Author Uri:
Template: twentysixteen
Version: 1.0
License: GNU General Public License v2 or later
Version: 1.0
*/

header#masthead {
padding: 0px 10px 0px 10px;
}
.entry-header {
display: none;
}
.site-branding {
margin-top: 0.1em;
margin-bottom: 0.1em;
}
.site-header-main {
max-height: 50px;
}
.site-header-menu {
margin-top: 0.1em;
margin-bottom: 0.1em;
}
div#page.site {
margin: _ 0 0 0;
}
.gallery {
margin: 0 auto;
text-align: center;
}
#page {
margin-left: auto;
margin-right: auto;
}’

This is driving me bonkers and I really wanted to fix it myself or find the answer listed somewhere but I am defeated. Thanks so much for any help and I’m really sorry for putting another damn “center image” problem in the forums.