[UI] Email invitation dark theme issues

[natalia-muzyka]

Preconditions
User has received an invitation email
Dark theme is set up on iOS

Steps to reproduce

1. Open an invitation email
2. Pay attention to the screen

Actual result:
1 - Mindlogger logo has a white background
2 - Button text is barely visible

Expected result:
1 - Mindlogger logo has a transparent background (as the CMI logo below in the footer)
2 - Button text is contrast with the button color (suggestion: make the text the same color as the text OR make the button color lighter)

Environment:
Apple iPad 9th gen (2021) / iOS 15.5