Hi,
1. There are many different places where you should check. First, check the VM general settings, then check the category one by one (because category settings can override the VM Global settings), third, check menu items that you assign to that category. Because, menu item can override all the settings. I know it looks complicated, but this is how VM and Joomla works, has nothing to do with MaterialMart.
If none of these works (it should work though), but lets say it didnt; then you can simply rename the "default.php -> default2.php" and "flexv2.php -> default.php" within this folder:
/templates/yYOUR_TEMPLATE/html/com_virtuemart/category/
so, flexv2 layout would become the default layout.
2- I can't checked your website. Your website has a FATAL ERROR that says:
Notice: Trying to get property of non-object in /data/web/virtuals/123447/virtual/www/domains/luky4.richta.eu/templates/jsn_shine_pro/niches/r2vf/html/mod_breadcrumbs/default.php on line 16
Fatal error: Call to a member function get() on null in /data/web/virtuals/123447/virtual/www/domains/luky4.richta.eu/templates/jsn_shine_pro/niches/r2vf/html/mod_breadcrumbs/default.php on line 17
3- This problem is related to this thread:
www.flexiblewebdesign.com/forum-virtuemart-template-materialmart-support/3807-1064-you-have-an-error-in-your-sql-syntax I believe you all settled it.
4- The REM measuring units in CSS is the modern way of measuring font-size through out the entire website. REM takes the font-size from the HTML tag, respectively. So, 1rem is what ever font-size your HTML element is using. It works like a percentage. 1rem is 100% of html element's font-size, 0.8 is 80% of html's font-size, 2rem is 200%.. etc..
To increase the font size, you can simply enter this CSS rule below (to any CSS file)
html {
font-size: 16px !important;
}
and magically, all font sizes gets enlarged. By the way, why in earth your html tag has the font-size:10px rule? Make sure to check your Joomla Template Settings, you might have accidentally enter 10px for the font-size. Because this is not normal. 10px is too small. Bootstrap uses 16px by default.
Regards.