You can define styles for the h1 - h6 tags, which goes some way to enforcing site style, by editing the cms 'HTMLEditor/fckconfig.js' file, some way down under '// Basic Block Styles (Font Format Combo).'
This has to be done in this format, ensuring that the 'punctuation' is correct, e.g:
'h1' : { Element : 'h1', Styles : { 'color' : 'Red', 'font-size' : '18px', 'text-decoration' : 'underline' } },
'h2' : { Element : 'h2', Styles : { 'color' : 'C1D0E1', 'font-size' : '14px', 'text-transform' : 'uppercase', 'font-weight' : 'bold' } },
'h3' : { Element : 'h3', Styles : { etc.
Back up your fckconfig.js file first if you have a go. You will probably have to clear your browser cache to see the changes.
As Jimmy says, you can only add classes, (classes begin with a dot, not a #) but you could add any new classes to your stylesheet:
.anewstyle {
color: #FFFFFF;
border: 1px solid #131D2B;
padding: 6px;
background-color: #2D4262;
}
and have them picked up by the Editor.