Fix session warning banner: visibility and dark theme.

[isoos]

• The session warning banner is displayed when the user session is close to expiry or was expired (on a page that may use the session cookies).
• This warning banner was broken for a while now, probably because of the change in the disposable banners. Adjusting the z-index to create a temporary fix there.
• The previous design had a yellow background, but I don't think we need that much color here, using the same background as the disposable banner use seems to be fine.
• Long-term fix may involve migrating this banner to use the same template and widget location as the disposable banners.

--