Certainly, you can play with the opacity of RadTooTip, and your approach is correct, however, the class you have selected to apply these settings to is incorrect. We do not have such CSS class (.Tooltip) at all.
The wrapping class of RadToolTips is coined by two strings - radtooltip + underscode + SkinName, i.e. - if you are using it with the Deafult skin, the class you have to apply your settings to will be .radtooltip_Default
As RadToolTip is rendered on the client and its markup is not visible with View Source, I recommend you to use FireBug
or Internet Explorer Developer Toolbar
in order to study the generated code and classes of our controls, these will certainly help you much with troubleshooting and modifications.
the Telerik team
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