I want the text in my submit buttons to become black, bold and underlined when hovering. So I programmed the buttons accordingly in Style Forms/Submit Buttons:/Default and Hover Style.
Default Style:

background-color: #dddddd;
color: #333333;
border: solid 1px #333333;
padding: 5px 5px 28px 5px;
font-size: 1.2em;
line-height: 1.5em;
height: 36px;
-moz-border-radius: 10px;
-khtml-border-radius: 10px;
-webkit-border-radius: 10px;
border-radius: 10px;

/* Shadow */
box-shadow: 10px 6px 15px #666666;
border-radius: 10px; -moz-border-radius: 10px;
-moz-box-shadow: 10px 6px 15px #666666;
-moz-border-radius:10px;
-webkit-box-shadow: 10px 6px 15px #666666;
-webkit-border-radius:10px;
-khtml-box-shadow: 10px 6px 15px #666666;
-khtml-border-radius: 10px;

Hover Style:

background-color: #dddddd;
color: #000000 !important;
text-decoration: underline !important;
font-weight:bold !important; 
border: solid 1px #333333;
padding: 5px 5px 28px 5px;
font-size: 1.2em;
line-height: 1.5em;
height: 36px;
-moz-border-radius: 10px;
-khtml-border-radius: 10px;
-webkit-border-radius: 10px;
border-radius: 10px;

/* Shadow */
box-shadow: 10px 6px 15px #666666;
border-radius: 10px; -moz-border-radius: 10px;
-moz-box-shadow: 10px 6px 15px #666666;
-moz-border-radius:10px;
-webkit-box-shadow: 10px 6px 15px #666666;
-webkit-border-radius:10px;
-khtml-box-shadow: 10px 6px 15px #666666;
-khtml-border-radius: 10px;

However, nothing happens when hovering as you can see here:
http://julesgobeil.com/gestion/?page_id=113

Strangely, changing elements in the Default Style do change the submit buttons but changing elements in the Hover Style do nothing.

Even more, changing elements in the Hover Style affect the Comments button as they should:

Submit Button Style.
Style the comment submit button, i.e. give it margin to move it around. This section here is specifically for the comment submit button. Additionally, default button styles will be applied, see menu tab "Forms", options "Submit Buttons: Default Style" and "Submit Buttons: Hover Style"

The Style And Configure Comments/Submit Button Style box is empty, so the Comments button takes all its parameters from the Submit Buttons configuration, Default and Hover. This button works as it should.
See here:
http://julesgobeil.com/gestion/?p=170#comments

Before you ask, there is nothing in Add HTML/CSS Inserts that can affect the buttons.

I can't understand ! Do you ?

Thanks
Jules

The styling in the theme options will affect SOME buttons depending on how they are created. In your case I would just add your code to CSS Inserts and use the selector for the button. The hover state selector should be something like

.button, .Button, input[type="submit"]:hover

Great ! It works fine! 1000 thanks...