[FIX] web_editor: properly detect background color in toolbar

When a background color is applied on an ancestor, while it's visibly
there, `getComputedStyle` doesn't know. Because it did depend on it,
the toolbar showed the wrong color in these cases.
Eg: `<span style="background-color: red;"><b>a[]bc</b></span>` was
showing no background color.

task-2778416

X-original-commit: a125933b34500d51183bc5eb02a531fb3b293de5
Part-of: odoo/odoo#86930