Thanks to Chad and Bartosz, https://gerrit.wikimedia.org/r/#/c/94276/ has been merged, and with the next MediaWiki core releases you should see two things:

1. the input box on the search page will be larger

2. the button will be a larger, blue button similar to the style seen on login, create account, and some other forms

You can see a screenshot of what this looks like at https://www.mediawiki.org/wiki/File:Search-UI-minor-tweak.png or by searching on Beta Labs (http://en.wikipedia.beta.wmflabs.org/)
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