The only way is using a custom script code in your website footer. First you will need to change one setting though, in order to make the search work with 0 input characters.
1. Go to the General Options -> Behavior panel and change the “Minimal character count to trigger search” option to 0
2. Add this script to your theme footer before the closing body tag: http://pastebin.com/raw.php?i=6hymZi4h
You can change the searchPhrase variable in that code if you want to use a default search phrase.
I do not recommend changing the CSS files directly as most of them are generated dynamically based on back-end values. Moreover updating the plugin will override all of your changes.
Most of the settings you can change on the Theme Options -> Isotopic results panel.
Rather changing the CSS I recommend adding your custom CSS code to the Theme Options -> Custom CSS panel. It's safe and does not get overridden by updates.
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