So I finally got the CSS code to “stick” using Firefox rather than Chrome, so (while I have custom CSS in other parts of my site, there was some problem interacting with your plugin).
However, the effect of the code is to move the search icon box away from the right hand side of the container in which it's placed, and that distance changes depending on the pixel count. However, the outlined icon box does not change in size. Interesting side note, with Chrome the search icon only shows, and with Firefox the search icon does not show, just the first three letters of “Search”. But in both cases the box is still square and has not lengthened.