[menu] Mobile navigation not sufficiently apparent

[UsmanAfzal]

URL: https://developers.openreferraluk.org/

Amend the border colour for the navbar button .navbar-dark .navbar-toggler to #ffffff and increase the border width to 2px.

Current

Change to

[UsmanAfzal]

@matt-porism This now only occurs on the subdomains:

• https://validator.openreferraluk.org/
• https://tools.openreferraluk.org/ApiQuery
• https://exporter.openreferraluk.org/

[matt-porism]

@UsmanAfzal I've released these apps with similar changes to those I made to the core developers site. Mobile menu border should be bolder, header image should not shrink on mobiles.