[FIX] mail: dark mode: text attachment readability

There is a specific rule that set the `o_AttachmentViewer_view`
background color to a static white color (i.e. `#FFFFFF`).
Then in dark mode, while the font color for the attachement "view"
is set to white due to the `color-scheme` meta, the background
keeps its white color, which makes the content unreadable. In this
commit we use instead the `$white` variable, which is properly
adapted in "dark mode", for the background color.

Steps to reproduce:
* Open Odoo
* Set Odoo in Dark mode using the dropdown menu of the user
* Open the Note app
* Make a note or open one
* Upload a JSON attachment in the chatter
* Open the preview of the attachment
  => BUG text in white and background color in white => text not visible

closes odoo/odoo#144763

X-original-commit: 2af9edb3ca45db0a36552ca751136b7b0c7b6a18
Signed-off-by: Pierre Paridans (app) <app@odoo.com>
Signed-off-by: Romeo Fragomeli (rfr) <rfr@odoo.com>

addons/web/static/src/core/file_viewer/file_viewer.scss

@@ -41,6 +41,6 @@
     outline: none;
 
     &.o-isText {
-        background: #FFFFFF;
+        background: $white;
     }
 }