Customize category layout list

Hikashop business 2.0
Joomla 2.5.15

I am using the Hikashop Conetent module to display my categories.
I have i set to Type of content: Category using Type of layout "List" enabled

I am using sub-categories so that the accordion function is also enabled.

Earlier I have done som customizing of this menu, but I want to customize it a little bit more.

Please see picture below to see what I mean.

1. I want to put a plus sign (image with a plus) in front of the main categories that has sub-categories.
So that when you click on the plus sign it drop down and you can see the sub-categories. How can I do this ?

2. I want the active sub-category to be displayd in "Bold" so that the user can see where he clicked. Is this possible to do ?

Thank you for reply.
I have tried to use the documentation, but it is very difficult because my CSS skills are very limited.
I have managed to transform the menu into the way I want it to look, so now I only need the plus/minus sign on the left side of the categories with sub-categories.

Please see image below to see how I need it to look.

.panel h3.pane-toggler-down a {
    background: url("../images/slider_minus.png") no-repeat scroll right top #F5F5F5;
    border-bottom: 1px solid #DDDDDD;
}

.panel h3.pane-toggler a {
    background: url("../images/slider_plus.png") no-repeat scroll right top #F5F5F5;
}

Hi,

To display plus or minus only on the categories which have sub categories, it will require more than css.
There is actually no way to differentiate this two kind of categories.

So a core file edition seems to be required.