css styles

[Arantor]

As for the confusion over :hover, it's not an attribute, it's a state. In your browser you have to toggle it on if you want to see it because there is no way a class makes sense for 'I am hovering over this element', but :hover isn't a class, and so is added/removed as needed.

[pocttopus]

What I was referring is that there was no displayed any attribute with hover option for any <tr> tag. 

[Gwenwyfar]

This is what I was talking about, to be sure we're on the same page:

By default, none of the states listed there will show up in the inspector. They are called pseudo-classes because they represent a state, they're not a class as such.

[Hatshepsut]

Is it possible to add hover color and in Message index and Board index tables?

[Arantor]

Sure, you just add a rule starting from those tables, then a tr:hover rule much as this one works. It's all just CSS...

[Gwenwyfar]

Hover can be added anywhere. The other pseudos can be added almost anywhere too (with some exceptions). You can find a list here.
(for the more adept, this is a more complete list, but not everything is usable)

Css is all about selecting things and doing something to it.