first step

__manifest__.py

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+# Copyright 2021 Akretion (https://www.akretion.com).
+# License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl).
+
+{
+    "name": "Custom Background",
+    "summary": "change background",
+    "version": "14.0.1.0.0",
+    "category": "Theme/Creative",
+    "author": " Akretion",
+    "license": "AGPL-3",
+    "application": False,
+    "installable": True,
+    "depends": [
+        "web_responsive",
+    ],
+    "data": [
+        "views/assets.xml",
+    ],
+}

readme/DESCRIPTION.rst

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+1. mettre une image dans static/src/img
+2. mettre son nom ligne 8 dans static/src/css/custom_background.scss
+   par exemple : background: url("../img/randonnee-montagne-tohapi.jpg");

static/src/css/custom_background.scss

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+/* License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl). */
+// customized background image
+.o_menu_apps {
+    user-select: none;
+
+    .dropdown-menu {
+        @include full-screen-dropdown();
+        background: url("../img/randonnee-montagne-tohapi.jpg");
+        background-size: cover;
+        background-position: center;
+
+    }
+}

views/assets.xml

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+<?xml version="1.0" encoding="utf-8" ?>
+<odoo>
+    <template
+        id="assets_backend"
+        name="Custom Background"
+        inherit_id="web.assets_backend"
+    >
+        <xpath expr="//link[last()]" position="after">
+            <link
+                rel="stylesheet"
+                href="/custom_background/static/src/css/custom_background.scss"
+            />
+        </xpath>
+    </template>
+</odoo>