generate default colors using theming colors

lib/Service/Color.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace OCA\SideMenu\Service;
+
+/**
+ * class Color.
+ *
+ * @author Simon Vieille <simon@deblan.fr>
+ */
+class Color
+{
+    /**
+     * @thanks https://stackoverflow.com/posts/54393956/revision
+     */
+    public function adjustBrightness(string $hexCode, float $adjustPercent): string
+    {
+        $hexCode = ltrim($hexCode, '#');
+
+        if (3 == strlen($hexCode)) {
+            $hexCode = $hexCode[0].$hexCode[0].$hexCode[1].$hexCode[1].$hexCode[2].$hexCode[2];
+        }
+
+        $hexCode = array_map('hexdec', str_split($hexCode, 2));
+
+        foreach ($hexCode as &$color) {
+            $adjustableLimit = $adjustPercent < 0 ? $color : 255 - $color;
+            $adjustAmount = ceil($adjustableLimit * $adjustPercent);
+
+            $color = str_pad(dechex($color + $adjustAmount), 2, '0', STR_PAD_LEFT);
+        }
+
+        return '#'.implode($hexCode);
+    }
+}