[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_bistro/views/images_library.xml

@@ -468,6 +468,11 @@
     <field name="name">website.s_kickoff_default_image</field>
     <field name="url">/theme_bistro/static/src/img/content/s_kickoff_default_image.jpg</field>
 </record>
+<record id="s_text_cover_default_image" model="theme.ir.attachment">
+    <field name="key">website.s_text_cover_default_image</field>
+    <field name="name">website.s_text_cover_default_image</field>
+    <field name="url">/theme_bistro/static/src/img/content/text_image.jpg</field>
+</record>
 
 <!-- Intro Pill -->
 <record id="s_intro_pill_default_image" model="theme.ir.attachment">

theme_bistro/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">
+        <h1 class="display-4">Present your<br/>menus.Easily.</h1>
+    </xpath>
+    <!-- Paragraph -->
+    <xpath expr="//p[hasclass('lead')]" position="replace" mode="inner">
+        <br/>Showcase your bistro’s menu online with a user-friendly platform that simplifies all the steps, from setup to menu management, making it easy for guests to explore and enjoy your offerings.<br/>
+    </xpath>
+    <!-- Button -->
+    <xpath expr="//a[hasclass('btn')]/t" position="replace" mode="inner">
+        Get started
+    </xpath>
+</template>
+
+</odoo>