I made several tries by creating new CSS classes and integrating them in: Plan> Presentation> Template & CSS> Class
That directly adds the specified CSS class to the plans container DIV element. Right click and inspect element on the plan to open your browsers developer tools and review the HTML structure carefully for CSS selectors. Several examples are already provided in that parameters description (e.g. cbreg_green). For example if I set that parameter to "myCSSClass" then it will result in the following HTML.
Before CSS Class
Code:
mb-3 align-top cbregPlan cbregPlan_rounded cbregPlanSelector cbregPlanSelector_rounded
After CSS Class
Code:
mb-3 align-top cbregPlan cbregPlan_rounded cbregPlanSelector cbregPlanSelector_rounded myCSSClass
So for your situation you'd end up writing your CSS as follows.
Code:
.cb_template .cbregPlan_rounded.cbregPlanSelector.myCSSClass {
display: inline-block;
width: 300px;
max-width: 100%;
}