I want the sidebars got an fixed height, NOT equal to the content to the post/page.
Just 400px, for example.
Which code in wich php i got to touch?
Thanks a lot
Pablo

you can't. The page is broken into three parts, the header, the middle and the footer
The middle is a table with one to five columns, the left column(s) the center area and the right column(s) - all teh columns in the row have to be the same height.

Maybe I expressed me wrong, I meaned how to set the widget´s background-color height of the sidebar equal to those widgets area, NOT equal to the content of the post/page. In other words, that only the page menu got a background color, not the whole sidebar.
Thank you

you can assign a background to the sidebar and/or
you can assign a background color to a widget container

if you assign a background to the sidebar it will cover the entire height of the sitebar
if you assign a background to the widgets, it will color all the widget containers in the sidebar

For the sidebars it is done at ATO->Style & configure SIDEBARS
For the widgets it is done at ATO->Style WIDGETS