OK. in your case you are using style# 4
find the file: /templates/rt_quasar_j15/css/style4.css
find this:/* Modules */
.icon1 .module-icon, .icon2 .module-icon, .icon3 .module-icon, .icon4 .module-icon {background-image: url(../images/body/style4/module-icons.png);background-repeat: no-repeat;}
you can replace it with something like this:/* Modules */
.icon1 .module-icon, .icon2 .module-icon, .icon3 .module-icon, .icon4 .module-icon {background-image: url(../images/body/style4/module-icons.png);background-repeat: no-repeat;}
.newstyle .rt-block {background: #000;}
.newstyle2 .rt-block {background: url(../images/backgrounds/style4/newstyle-bg.jpg) 50% 100% repeat-x;}
Then for which ever module you choose add the newstyle or newstyle module class suffix in the module parameters (found in module manager for that specific module)
two things:
1. for suffix 'newstyle' change the color code #000 to your taste
2. for suffix 'newstyle2' make sure you have placed the background image in the palce your code refers to.
Of course now that you know where to look you can take it to whatever level of creativity you choose.