[FIX] theme_*: fix s_text_cover design

This commit has two main goals:

1. Adapt the copy of the `s_text_cover` snippet to fit each theme

2. Introduce custom images for each themes, as the `s_text_cover` was
   always using the same image until now

This commit also fixes some conflicting classes due to a change of
`background-color` in the default snippet.

task-4119904
Part of task-3619705

Part-of: odoo/design-themes#919
Related: odoo/odoo#180736
Signed-off-by: Benjamin Vray (bvr) <bvr@odoo.com>
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

theme_notes/views/snippets/s_text_cover.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<odoo>
+
+<template id="s_text_cover" inherit_id="website.s_text_cover">
+    <!-- Title -->
+    <xpath expr="//h1" position="replace" mode="inner">
+        Share Music.<br/>Effortlessly.
+    </xpath>
+    <!-- Paragraph -->
+    <xpath expr="//p[hasclass('lead')]" position="replace" mode="inner">
+        <br/>Showcase your band’s music and updates online with a user-friendly platform that simplifies everything, from setup to track management, making it easy for fans to discover and enjoy your sound.<br/>
+    </xpath>
+    <!-- Button -->
+    <xpath expr="//a[hasclass('btn')]/t" position="replace" mode="inner">
+        Get started
+    </xpath>
+</template>
+
+</odoo>