Notifications Bar on home page only

Using the very example from the Codex in the Settings API section. Put it in the theme’s functions.php:

add_action('admin_init', 'eg_settings_api_init');

function eg_settings_api_init() {
    add_settings_section(
        'eg_setting_section',
        'Example settings section in reading',
        'eg_setting_section_callback_function',
        'reading'
    );

    add_settings_field(
        'eg_setting_name',
        'Example setting Name',
        'eg_setting_callback_function',
        'reading',
        'eg_setting_section'
    );

    register_setting('reading','eg_setting_name');
}

function eg_setting_section_callback_function() {
    echo '<p>Intro text for our settings section</p>';
}

function eg_setting_callback_function() {
    echo '<input name="eg_setting_name" 
        id="gv_thumbnails_insert_into_excerpt" 
        type="checkbox" 
        value="1" 
        class="code" ' 
        . checked( 1, get_option('eg_setting_name'), false ) 
        . ' />Explanation text';
}

This creates a checkbox in Settings > Reading:

And in the theme header.php file, it can be used to render a big red warning banner:

<?php 
    if( get_option('eg_setting_name') ) {
        echo '<h1 style="background-color: #fcc;width:100%;height:3em;line-height:3em;text-align:center;top:40px;position:fixed">
            Warning!
            </h1>';
    }
?>