How to reduce the size of the white box at the bottom of pages

how to reduce the size of the white box at the bottom of pages where “older posts” is written.

Hello @sksalvatore,

Please try to use the following CSS code.

You can add CSS code in Dashboard → Appearance → Customize → Additional CSS (WordPress 4.7 and up).

nav.navigation.posts-navigation {
    padding-top: 10px;
    padding-bottom: 10px;
}

Please feel free to ask any other questions that you might have.

Kind Regards, Roman.