You have a white background in your campaign and the checkboxes are also white and therefore barely visible ?
This issue is due to the fact that you have either not selected any background color or a white background color for your buttons. Elements in your campaign such as checkboxes, inputs hovering effect, asterisks for mandatory fields, ... will use the same color as the color set for the background color of your buttons.
If you really need to use a white background in your contest with white or non-stylished buttons, then, there is a CSS workaround !
Copy the following code and paste it in the step 2. Look and feel, then in the tab CSS :
/* Default buttons color */
.flat_radio, .flat_checkbox { border-color: #b30808; }
/* Hover & Active buttons color */
.flat_inside_radio:after { background: #ff0000; }
.flat_inside_checkbox:after, .flat_radio input[type=radio]:checked + .flat_inside_radio:before, .flat_checkbox input[type=checkbox]:checked + .flat_inside_checkbox:before { border-color: #ff0000; }