improve Google ranking

Checking your website with Google's own PageSpeed Insights tool reveals the render-blocking JavaScript and CSS being referred to.
They are gallery and theme related external JavaScript and CSS files which are necessary for the functionality and styling of the content above-the-fold. (In a typical Showkase page, pretty much everything is above-the-fold.)

Suggestions to defer or asynchronously load them might not help much as your above-the-fold content would not look the way it should until the files are fully loaded anyway and there is not a great deal of code on a typical Showkase page that the render-blocking elements would be holding up.

If you like, you could certainly try deferring the loading of the theme related JavaScript files.
Open the showkase/_themes/base/pagetypes/basetheme.tpl file in a plain text editor and add the defer attribute to all <script> tags between lines 33 and 44 (the 'scriptlinks' section). Please note that the line numbers refer to the current version of Showkase (v1.6.1).
However, please be aware that this is not officially supported or tested and might even break your Showkase pages. (For example, if you defer the loading of the 'juicebox.js' gallery file in a Juicebox gallery page, the gallery will simply not display. The 'juicebox.js' gallery file cannot be deferred or loaded asynchronously. The file needs to have been loaded synchronously before the JavaScript embedding code is executed.)
It is certainly something that you could try, though. It is easy to revert if it does not help.
(Be sure to re-publish your site after making any modifications to source files.)

The other suggestion to inline the code would probably also not help much in real terms and would require the structure of Showkase to be changed significantly.
The render-blocking JavaScript and CSS is mostly theme related and I expect that restructuring Showkase to inline the theme code (rather than have it modular in discrete external files) would make maintaining themes and creating new ones a much more difficult process.

Just for the record, I picked a couple of websites at random (youtube.com and wordpress.org), analyzed them with the test website you quoted and got exactly the same message that you reported. It does not seem to be an not uncommon occurrence.

Incidentally, the test website's own pop-up results page (after clicking 'See what to fix') does not fit within my laptop's screen and I have to press F11 to go fullscreen to see the close button. Even though their own web page passes all their own tests, usability is perhaps more important than ultimate speed (although it would be nice to have both in an ideal world).