[IMP] theme_clean: update the theme

This commit updates the theme to improve the design with new colors, new
pictures and new snippet customizations.

task-2597387

X-original-commit: e6f1c3bb12
Part-of: odoo/design-themes#501

theme_clean/views/snippets/s_banner.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<odoo>
+
+<template id="s_banner" inherit_id="website.s_banner">
+    <!-- Filter -->
+    <xpath expr="//div[hasclass('container')]" position="before">
+        <div class="o_we_bg_filter bg-black-50"/>
+    </xpath>
+</template>
+
+</odoo>