Asteroid Theme only has two columns, a content area and a sidebar area. This makes it much easier to make the layout adapt to various screen sizes.
Frontier Theme on the other hand has many possible layout configurations to choose from: one, two, three column layouts and page templates that follow their own column layouts. In addition to this, the columns have widths that can be set by the user. So there are a lot more variables to consider.
Anyway, since your chosen layout is composed of just two columns, you can add this code on Frontier Options -> CSS
This should make the columns behave similar to Asteroid Theme.
#content, .sidebar {
display: table-cell;
float: none !important;
vertical-align: top;
}