[FIX] website: make the "Text Cover" snippet look good in grid mode

Before this commit, toggling the "Text Cover" snippet to grid mode did
not produce a good-looking result: the column containing the background
image loses all its padding and therefore appears smaller and shifted
compared to the normal mode.

This commit adds a background color to the image column. Indeed, with a
background color, and more precisely thanks to the `o_cc` class, the
padding is taken into account when computing the grid items size, making
them look similar to how they were in normal mode.

task-3369695

Part-of: odoo/odoo#136683

addons/website/views/snippets/s_text_cover.xml

@@ -10,7 +10,7 @@
                     <p class="lead">Write one or two paragraphs describing your product, services or a specific feature. To be successful your content needs to be useful to your readers.</p>
                     <a t-att-href="cta_btn_href" class="btn btn-primary"><t t-esc="cta_btn_text">Contact us</t></a>
                 </div>
-                <div class="o_not_editable pt160 pb160 oe_img_bg col-lg-7 d-none d-md-block" style="background-image: url('/web/image/website.s_text_cover_default_image'); background-position: 100% 0;"/>
+                <div class="o_not_editable o_colored_level o_cc o_cc1 pt160 pb160 oe_img_bg col-lg-7 d-none d-md-block" style="background-image: url('/web/image/website.s_text_cover_default_image'); background-position: 100% 0;"/>
             </div>
         </div>
     </section>