Apply white color to button (#1375)

-   Ticket: [ENG-3241](https://openscience.atlassian.net/browse/ENG-3241)
-   Feature flag: n/a

## Purpose

Apply the white color to the sign up button text so that, when the jbp css is modified, the button will have white text.

## Summary of Changes

1. Apply white color to text of button

## Side Effects

No, this should be isolated.

## QA Notes

Test on https://staging.osf.io/collections/test/discover - it should work with no other changes.

Author: brianjgeiger

app/styles/_components.scss

@@ -428,6 +428,7 @@
     padding: 4px 13px;
     margin-top: 3px;
     font-size: 15px;
+    color: #fff;
 }
 
 #navbarScope .btn-top-login {