Making a coulour theming with the change style dialog means that You have to choose the attribut which should get a colour theming. When choosing the method of classifying the attribute values (double type) the labels often breaking in decimal numbers with lots of decimal places. That is ugly for layouts. At the moment the only way is to edit this lable table manually and this takes allways time.
A better way should be an option to approximate the numbers for the labels (e.g. with a default of 2 decimal places).