Change css for tag in wordpress admin

If you’re trying to change the styles of the wysiwyg editor, which I assume you are based on your code example, you need to use the add_editor_style() function.

As per the example in the code reference.

Step 1

Add the following to the functions.php file of your theme.

/**
 * Registers an editor stylesheet for the theme.
 */
function wpdocs_theme_add_editor_styles() {
    add_editor_style( 'custom-editor-style.css' );
}
add_action( 'admin_init', 'wpdocs_theme_add_editor_styles' );

NB The stylesheet is relative to theme root.

Step 2

Next, create a file named custom-editor-style.css in your themes root
directory. Any CSS rules added to that file will be reflected within
the TinyMCE visual editor. The contents of the file might look like
this:

#tinymce p {
    margin: auto;
}

For other p tags you could try placing some inline style to the admin_head. Like so,

// Add inline CSS in the admin head with the style tag
// put this in to your theme's functions.php
function my_custom_admin_head() {
    echo '<style>p {margin: auto;}</style>';
}
add_action( 'admin_head', 'my_custom_admin_head' );

NB Also make sure the css selectors are correct. I didn’t check them for these examples.

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