[fix/branded-login-ipad-UI] Fixed Branded UI on iPad

[hosy]

Description

1. UI fix for branded login on the iPad
2. Fill color for branded button was not used

Related Issue

1. https://github.com/owncloud/enterprise/issues/4367
2. https://github.com/owncloud/enterprise/issues/4366

Motivation and Context

Better UI experience

How Has This Been Tested?

• Use a branded Client
• Test UI on iPad
• Test UI on iPhone

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[jesmrec]

• Reduced width of the Continue button ✅
• Branded colours of Login button ✅

Approved

CC @hosy