how to change color of entire page / container?

Do you want that white inner container to be black as well? IF so, then all the font colors have to be changed, too.

.main-container {
    background: black;
}
body.boxed .site-main {
    background: black;
}
/* now fix up fonts */
a {
    color: gray;
}

To add or override CSS: use the “Additional CSS” option in the customizer. https://codex.wordpress.org/CSS#Custom_CSS_in_WordPress

Learn to use the Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS.

add “, a:hover” to the CSS selector where you set the link color. That is, it should be “a, a:hover”.