Improvements to event registration modal

We have updated a few details on the event's registration modal. Here's what's new:

We have updated the placeholder texts and text opacity to make them easy to read and use
Added a dropdown arrow next to the affiliation field to make it easier for users to click and select from the available options
Updated the text and text color of the error messages to enhance readability
Changed the text color of the input values provided by the user to clarify the distinction between placeholder text and input value text