It's likely a built in limitation due to using the web browser instead of FileMaker Pro as your "client application". I don't do a lot of IWP so I've hesitated to suggest anything as an answer to this question. Conditional formatting would seem a possibility, but this feature is also not supported in IWP.
There is, however, an "old school" method for highlighting a field that does work in IWP. It's how we did things in FileMaker before conditional formatting became possible. You may be able to get this method to work for you.
Add container field to your database system. Either define it as a global field or put it in a table where you can access it via a relationship that uses the X (cartesian join) operator. Enter layout mode and draw a small rectangle with a light "pastel" fill color. Either specify zero line width or select the same line color as you have a fill color. Copy it to the clipboard. Enter browse mode and paste it into your container field.
Now, in your portal's table, define a calculation field and specify "container" as its return type. Give it an expression such as:
If ( $$RecordID = Get ( RecordID ) ; ContainerField ) // use the correct syntax for referring to the container field into which you just pasted a rectangle.
If ( $$SerialID = PrimaryKeyID ; containerField )
With the first option, the field must be an unstored calculation.
Place this field in your portal row behind the other fields, which must be transparent. Resize it to fill the entire portal row. Use data formatting options in the inspector to specify that this calcuation field "reduce or enlarge to fit" and clear the "maintain original proportions" check box.
Now use a script to set the global variable to either Get ( RecordID ) or YourTable::PimaryKeyID to highlight the current portal row.
But please note that I have not actually tried to implement this method in IWP. On paper it should work, but try it out and let me know if it does.