Peplace "button" class in the WooCommerce templates to "btn btn-default" class. Useful if you can use Bootstrap Theme.

wc2bs-patcher.php

<?php

function ends_with($haystack, $needle)
{
    $length = strlen($needle);
    if ($length == 0) {
        return true;
    }

    return (substr($haystack, -$length) === $needle);
}

function get_all_files($directory) {
    if (!is_dir($directory)) {
        throw new Exception("Is not a directory: $directory");
    }
    $dir = opendir($directory);
    if (!$dir) {
        throw new Exception("Unknown problem. Directory: $directory");
    }
    $ret = [];
    while (($file = readdir($dir)) !== false) {
        if ($file == '.' || $file == '..') {
            continue;
        }
        if (!ends_with($directory, '/')) {
            $directory .= '/';
        }
        $file_name = $directory . $file;
        if (is_dir($file_name)) {
            $ret = array_merge($ret, get_all_files($directory . $file));
        } else {
            $ret[] = $directory . $file;
        }
    }
    return $ret;
}

define('PATTERN', '/\sclass="(([^"]*)\s)?(button)([^"]*)"/');
define('PEPLACEMENT', ' class="$1btn btn-default$4"');

echo "Woocommerce Template Button Patcher\n";
echo "Directory: {$argv[1]}\n";
$files = get_all_files($argv[1]);

foreach ($files as $file) {
    echo "File: $file";
    $content = file_get_contents($file);
    $count = 0;
    $new_content = preg_replace(PATTERN, PEPLACEMENT, $content, -1, $count);
    if ($count > 0) {
        echo " REPLACED";
        file_put_contents($file, $new_content);
    }
    echo "\n";
}