Hi,
This is actually our future plan. We are developing a dashboard to choose a theme and customize colors for certain elements.
But for now, you should manually replace the class-names to change the colors. To see color-palatte and the class associations; see this page:
materializecss.com/color.html
For instance, let's assume you want to change the color of "Add to Cart" button from "red" to "blue", simply, open this PHP file:
YOUR_TEMPLATE//templates/t3_bs3_blank/html/com_virtuemart/sublayouts/addtocartbtn.php
and where ever you see the word "red", replace it with: "blue", "blue lighten-1" or "blue darken-1".. And boom, the add to cart button color will be changed.
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The same idea can be applied to everywhere. You just need to find the proper file to replace the class names. You should better start using "Chrome Developer Tools", which will save you lots of time.
Regards,
Support