By default, the platform does not have this option. You can use the following function code.

Please follow the steps below:

1. Go to Appearance > Theme Editor.

2. Under Select theme to edit, choose (BuddyBoss Child/whatever theme is active), then click Select.

3. Below Theme Files, select Theme Functions (functions.php).

4. Append the code just before the closing PHP tag “?>”:

function bb_login_custom_css() { ?>
<style>
 .login #login #login_error {
background-color: #ddd;
border-left-color: #ddd;
color: #000;
}
</style>
<?php }
add_action( 'login_enqueue_scripts', 'bb_login_custom_css');

You may change the colors. 

5. Click Update File to save the changes.

Note: Any modifications are considered as custom work already. Know more about our Support Policy here: https://www.buddyboss.com/support-policy/

Screenshot