Add argon theme

luci-theme-argon/htdocs/luci-static/argon/custom.css

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+:root {
+    --main-color: #09c;
+    --header-bg: #09c;
+    --header-color: #fff;
+    --bar-bg: #5e72e4;
+    --menu-bg-color: #fff;
+    --menu-color: #5f6368;
+    --menu-color-hover: #202124;
+    --main-menu-color: #202124;
+    --submenu-bg-hover: #d4d4d4;
+    --submenu-bg-hover-active: #09c;
+
+    --blue: #5e72e4;
+    --indigo: #5603ad;
+    --purple: #8965e0;
+    --pink: #f3a4b5;
+    --red: #f5365c;
+    --orange: #fb6340;
+    --yellow: #ffd600;
+    --green: #2dce89;
+    --teal: #11cdef;
+    --cyan: #2bffc6;
+    --white: #fff;
+    --gray: #8898aa;
+    --gray-dark: #32325d;
+    --light: #96999c;
+    --lighter: #e9ecef;
+    --primary: #5e72e4;
+    --secondary: #f7fafc;
+    --success: #2dce89;
+    --info: #11cdef;
+    --warning: #fb6340;
+    --danger: #f5365c;
+    --dark: #212529;
+    --default: #172b4d;
+    --white: #fff;
+    --neutral: #fff;
+    --darker: black;
+    --breakpoint-xs: 0;
+    --breakpoint-sm: 576px;
+    --breakpoint-md: 768px;
+    --breakpoint-lg: 992px;
+    --breakpoint-xl: 1200px;
+
+    --font-body: "Microsoft Yahei", "WenQuanYi Micro Hei", "sans-serif", "Helvetica Neue", "Helvetica", "Hiragino Sans GB";
+}