Basically it's because the modules have thier own classes applied by DNN to them, so your applying your :hover and such to classes that aren't being used inside the module. You're applying thm to the cell, but in the cell are more cells that have thier own class.
So...There is a file in DotNetNuke, called default.css that applies to everything in your website. Every class in that file can be overridden. To override them, in your skin.css file you must declair the same class that your overriding, and set the attributes you wish to override.
make sure, when you override something, override it exactly. If they say border-left you say border-left adn so on.
Here is the section you need to override. I've highlighted the items that you must override. Just put those classes in your skin.css file and it should work. if not take a look at default.css in your skin admin, site settings page...you'll see it all there.
/* GENERIC */
H1
{
font-family: Tahoma, Arial, Helvetica;
font-size: 20px;
font-weight: normal;
color: #666644;
}
H2
{
font-family: Tahoma, Arial, Helvetica;
font-size: 20px;
font-weight: normal;
color: #666644;
}
H3
{
font-family: Tahoma, Arial, Helvetica;
font-size: 12px;
font-weight: normal;
color: #003366;
}
H4
{
font-family: Tahoma, Arial, Helvetica;
font-size: 12px;
font-weight: normal;
color: #003366;
}
H5, DT
{
font-family: Tahoma, Arial, Helvetica;
font-size: 11px;
font-weight: bold;
color: #003366;
}
H6
{
font-family: Tahoma, Arial, Helvetica;
font-size: 11px;
font-weight: bold;
color: #003366;
}
TFOOT, THEAD
{
font-family: Tahoma, Arial, Helvetica;
font-size: 12px;
color: #003366;
}
TH
{
vertical-align: baseline;
font-family: Tahoma, Arial, Helvetica;
font-size: 12px;
font-weight: bold;
color: #003366;
}
A:link
{
text-decoration: none;
color: #003366;
}
A:visited
{
text-decoration: none;
color: #003366;
}
A:hover
{
text-decoration: underline;
color: #ff0000;
}
A:active
{
text-decoration: none;
color: #003366;
}
http://www.microsoft.com/downloads/details.aspx?familyid=e59c3964-672d-4511-bb3e-2d5e1db91038&displaylang=en <--I like this one for finding elements
http://chrispederick.com/work/web-developer/ <-- this one is GREAT for testing CSS.