change button colors

Go to your theme CSS and insert this

#wpc-col-1 ul li a:hover, #wpc-col-1 ul li.active-wpc-cat a {background: #0f75bc !important;}

Change the numbers and letters from #0f75bc to whatever combination makes red (that one is for blue). Maybe someone else can tell you exactly what makes red.

Thanks for solution.

Is there also one for the square shown when you′re clicking on the thumbnail?

Yep, change the colour of the border on the image rollovers with this:

.wpc-img:hover {
	border: 5px solid #B22222 !important;
}

brilliant. worked like a charm.

Thumbs up for great plugin and awesome support

No worries, happy to help ??

Most browsers have a way to find the css rules for a specific item on a page (in your case the category button. In Goggle Chrome you right click and select Inspect Element; in Internet Explorer use the Property Inspector; in Firefox right click and select Inspect Element. Firebug is a nice plugin too.