Adjusting header text

#site-logo {
   width: 50%;
}

takes care of the title.

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 to help you see and test changes to your CSS.

For the gallery question, I recommend asking at https://wordpress.org/support/plugin/nextgen-gallery#new-post so the plugin’s developers and support community can help you with this.

#site-logo {
    width: 100%;
    text-align: center;
}
#site-logo .site-title {
    font-size: 4rem;
    line-height: 1;
    letter-spacing: initial;
}