For Vertical menu you need to change skin.xml as for horizontal the default skins read from default values. look around and you will find some free skins with vertical menu so just go ahead study them.

        <Setting>
                <Name>Display</Name>
                <Value>vertical</Value>
            </Setting>

with regards to the css changes, if it is not in skin.css then it picks the classes up from DotNetNuke\Portals\_default\default.css. So if you want to change it then copy it from there and paste it in skin.css with the necessary changes. Skin.css overrides the one in default directory.

Salar

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If I am to understand the original question correctly, I am also having the same problem. I just want to change the font color and size of the links in the links module, which as far as I can tell is a treeviewmenu control. I did pull the suggested styles out of the default style sheet and put them in skin.css, but it still doesn't work. I just need to change the color and size of the text for those links. Any suggestions?