How to align bold,italic and underline in wp-admin?

If it is editor mode specific, then make a CSS file named: editor-style.css and then add the following code into functions.php within PHP tags:

// CUSTOM EDITOR STYLES
// This theme styles the visual editor with editor-style.css to match the theme style.
add_editor_style();

Now put some CSS code to match the site’s CSS into the editor.