[FIX] mail: strip bg-o-color class from body_html

When a mass mailing block uses a `bg-o-color-N` theme color class, the mailing's `body_arch` stores the class and `convert_inline` correctly inlines the resolved color into `body_html`, matching the website palette at save time. The class also stays on the element in `body_html`. The stylesheet rule that gives `bg-o-color-N` its color is declared with `!important`. When the website palette is later rebuilt (any change to the primary colors), the new `bg-o-color-N` rule wins over the inline color whenever `body_html` is rendered. So an already-sent mailing reopened in the backend shows the new palette's color instead of the one that was picked, and resaving the mailing bakes that new color into `body_html`.

`classToStyle` already does the right thing for the property value. What was missing is dropping the class itself from `body_html` once its style has been inlined, so no future `!important` palette rule can override the inline color. `body_arch` keeps the class, so the editor preview stays theme-aware while editing, but `body_html` is now stable across palette rebuilds.

Steps to reproduce:
1. Open Email Marketing and create a new mailing using the Welcome Message template
2. Select a content block, open Customize, set the background to the 5th theme color
3. Save the mailing
4. Open the Website editor and change the 5th primary color to a different value
5. Reopen the saved mailing in the backend

=> the block's rendered background follows the new website color instead of the one picked at design time

Ticket [link](https://www.odoo.com/odoo/project.task/5892350)
opw-5892350

closes odoo#268001

X-original-commit: 880fe06
Signed-off-by: Damien Abeloos (abd) <abd@odoo.com>
Signed-off-by: Mohamed Jemai (mojem) <mojem@odoo.com>

Author: Alpha9584

addons/mail/static/src/views/web/fields/html_mail_field/convert_inline.js

@@ -43,6 +43,7 @@ const RE_PADDING_MATCH = /[ ]*padding[^;]*;/g;
 const RE_PADDING = /([\d.]+)/;
 const RE_WHITESPACE = /[\s\u200b]*/;
 const SELECTORS_IGNORE = /(^\*$|:hover|:before|:after|:active|:link|::|')|@page/; // :not(:has) should be legal
+const RE_THEME_COLOR_CLASS = /^bg-o-color-\d+$/;
 const CONVERT_INLINE_BLACKLIST_CLASSES = ["o_mail_redirect"];
 // CSS properties relating to font, which Outlook seem to have trouble inheriting.
 const FONT_PROPERTIES_TO_INHERIT = [
@@ -569,6 +570,18 @@ export function classToStyle(element, cssRules) {
             });
         }
 
+        const themeColorClasses = [...node.classList].filter((c) => RE_THEME_COLOR_CLASS.test(c));
+        if (themeColorClasses.length) {
+            writes.push(() => {
+                for (const cls of themeColorClasses) {
+                    node.classList.remove(cls);
+                }
+                if (!node.classList.length) {
+                    node.removeAttribute("class");
+                }
+            });
+        }
+
         if (node.nodeName === "IMG") {
             writes.push(() => {
                 // Media list images should not have an inline height

addons/mail/static/tests/inline/convert_inline.test.js

@@ -888,6 +888,23 @@ describe("Convert classes to inline styles", () => {
         styleSheet.deleteRule(0);
     });
 
+    test("strip theme color classes after inlining their styles", async () => {
+        const bgColor = "rgb(17, 24, 39)";
+        const style = document.createElement("style");
+        style.textContent = `.bg-o-color-5 { background-color: ${bgColor} !important; }`;
+        const fixture = getFixture();
+        fixture.append(style);
+
+        editable.innerHTML = `<div class="bg-o-color-5 keep-me">Hello</div>`;
+        fixture.append(editable);
+
+        classToStyle(editable, getCSSRules(editable.ownerDocument));
+
+        const block = editable.querySelector(".keep-me");
+        expect(block).toHaveStyle({ backgroundColor: bgColor });
+        expect(block).not.toHaveClass("bg-o-color-5");
+    });
+
     test("simplify border/margin/padding styles", async () => {
         // border-radius
         styleSheet.insertRule(