Child Themes of Non-WordPress Sanctioned Templates

Use get_stylesheet_directory_uri() and get_stylesheet_directory() for Child Themes

Use get_template_directory_uri() and get_template_directory() for Parent Themes

Thus making your function:

<?php if ($gridly_color_scheme == 'dark') { ?>
    <link rel="stylesheet" href="https://wordpress.stackexchange.com/questions/107986/<?php echo get_template_directory_uri(); ?    >/css/dark.css" /> 
<?php  } elseif ($gridly_color_scheme == 'custom') {  ?>
    <link rel="stylesheet" href="<?php echo get_stylesheet_directory_uri(); ?>/css/custom.css"/> 
<?php  } else {?>
     <link rel="stylesheet" href="<?php echo get_template_directory_uri(); ?>/css/light.css"/> 
<?php  } ?>

…subtle difference which can be easy to miss.

Helpful resources:

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