[FIX] web: properly display table texts over any background

Bootstrap tables can basically be customized with the `$table-bg` and
`$table-color` variables. The problem is that, by default, BS4 defines
them so that the background-color is null (so transparent: displaying
the background-color of its ancestors) but the color is forced to the
body color (by default: white). This is a problem as soon as the
ancestors background colors are a color close to the body text color:
the text becomes invisible. For instance, in website:
- Set a body background color to black, the body text will automatically
  become white.
- Add a table in a snippet: still ok, the text in the table is white
  over the black body (the table being transparent).
- Then set the snippet background to white -> the table text will still
  be white... but now over a white background.

This should be reviewed in master: it should be ok to set the variable
$table-color to `null` thus letting the table be transparent and have
the same text color as its parent. But in stable, changing a color
variable to `null` could break customizations relying on the fact this
is a set color. It would also not make sense if the user set up a
`$table-bg` value going well with table text forced to the body color.
Instead, here, in the very specific case we have a transparent table bg
and table color equal to the body color, we temporarily unset the table
color variable for the duration of the bootstrap table rules.
Note: we cannot create a rule in an "Odoo file" to fix this as unsetting
the color for the `.table` rule would also unset the color in the case
of a `.table.bg-XXX` where we still want `.bg-XXX` to force the color.

task-2728923
opw-3048306
opw-3180568

closes odoo/odoo#114631

X-original-commit: 02c2cfdff7c29252c7036e587b12d82442906e59
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/web/static/src/scss/bootstrap_overridden_frontend.scss

@@ -44,6 +44,19 @@ $blockquote-small-color: inherit !default;
 // Link
 $link-decoration: none !default;
 
+// Tables
+//
+// Customizes the `.table` component with basic values, each used across all table variations.
+
+// Bootstrap tables can basically be customized with the `$table-bg` and
+// `$table-color` variables. The problem is that, by default, BS5 defines them
+// so that the background-color is null (so transparent: displaying the
+// background-color of its ancestors) but the color is forced to the body color
+// (by default: white). This is a problem as soon as the ancestors background
+// colors are a color close to the body text color: the text becomes invisible.
+// We disable this behavior by not forcing a text color for tables.
+$table-color: inherit !default;
+
 // Forms
 
 $input-transition: background-color .05s ease-in-out, border-color .05s ease-in-out, box-shadow .05s ease-in-out !default;