Hi Peter,
I would suggest to change the background-color of the td and not the div.
The way you set it up in your css is: "Selects every <td> element whose title attribute value begins with "-".
On which element did you place the class .customcell?
If the .customcell is placed on the td element then the code would be:
td.customcell[title^="-" ] div {
color: white;
background-color: red;
}
What do you mean with a certain range?