You can use add_editor_style()
to override the default styles with your custom ones. Like this:
function wpse158918_override_default_tinymce_styles() {
add_editor_style( 'your-editor-styles.css' );
add_action( 'after_setup_theme', 'wpse158918_override_default_tinymce_styles' );
The code would go into your functions.php
, the path to the stylesheet is relative to the directory of your current theme. If you are not doing this from your theme, but a plugin, then use the mce_css
filter. Example from codex page:
function plugin_mce_css( $mce_css ) {
if ( ! empty( $mce_css ) )
$mce_css .= ',';
$mce_css .= plugins_url( 'editor.css', __FILE__ );
return $mce_css;
}
add_filter( 'mce_css', 'plugin_mce_css' );