Running SMF Arcade v2.7.0.8 on SMF 2.1.7 shows a few 404 errors when browsing a forum on URLs like:
Quote/Themes/default/css/arcade-special.css?v2708
Checking the code this file is referred via /Sources/ArcadeHooks.php:
<link rel="stylesheet" href="' . $settings['default_theme_url'] . '/css/arcade-special.css?' . $suffixVersion . '">
but not included in the Download / install archive for unknown reasons.