Middle content should fill viewport

**Describe the bug**
Middle content should flex to fill entire viewport

**To Reproduce**

1. Go to login screen.
2. Note the middle content.

**Expected behavior**

Middle content should flex to fill entire viewport

**Screenshots**

[from our a11ydocs 11 March meeting](https://docs.google.com/document/d/1YxgGP9cyG9ZgAnlGxPbnDPVLhes3kZsE2k876_5rQJ0/edit?tab=t.0#bookmark=id.sysywpbg7v9g)