Conditional formatting on multiple report items is supported i.e. you can select several TextBoxes and click on the ConditionalFormatting in the property grid to open the formatting dialog where you would apply the desired condition. This way the formatting rule would be applied to all selected items and you do not have to define it separately for each and every item.
While you can't reuse one conditional formatting from the report designer once it has been defined, you can do so by manually creating the formatting rule in the report constructor, this way you can always add the formatting rule to the ConditionalFormatting collection of any new item you need.
As you're already utilizing an user function, you can include the ReportItem as argument to that function and get its value. Then if the value is 'Yes', you could return an appropriate color and use the user function as Value for a binding expression. For sample implementation, please check the ProductCatalog demo report where we use ColorFromName user function to specify a color via Bindings
There is definitely a room for improvement by changing how styles are applied, so that an already defined StyleName can be used in binding or even in a conditional formatting, which would make it really reusable. These improvements are already logged in our system and would be considered for subsequent version of the product.
All the best,
the Telerik team