What exactly do you mean by "show only border?" You only want to see the border around a control based on conditional formatting? Can you be a little more specific?
From the screenshot it looks like you are trying to highlight certain rows by showing only a border around them, rather than shading the background. Access conditional formatting does not give you direct control over the border style of a control, only the fill color, font color, etc. If you need the effect of just a border, the usual workaround is to use a box control or an unbound rectangle and make it visible when the condition is met. Another approach is to set the back color and fore color the same so it looks like the control is blank, then rely on the border to stand out.
Access conditional formatting does not give you that kind of control over borders in a continuous form. You can change back color, fore color, and font styles, but not the border. If you want to highlight a row with only a border, you would need a workaround, like adding a box or rectangle control behind the fields and toggling its visibility or color based on your condition.