Setting backgroundColor transparent in Sideloaded Captions
I am making a private channel in which movies are captioned via a sideloaded SMPTE-TT file like this. I have found by experiments with my Roku channel that I can vary the captions' text color by changing a parameter (tts:color="red", or whatever) in the two places it occurs in the file's preamble.
I would like to make the captions' background color transparent. I have tried adding tts:backgroundColor="transparent" [ and also tts:backgroundColor="off" and tts:backgroundColor="rgba(0,0,0,0)"] in two places in the preamble. I have also tried adding these text strings as a style override to a span inside one caption line. None has worked: the captions' background color remains black with an opacity of [it looks like maybe about] 80%.
In Roku's Home > Settings > Captions, I have Mode = On and all other parameters set to default. If I set Background Opacity to Off, then I get what I want. My problem is that I would like to control that feature from the sideloaded captions file, if the viewer will permit.
Below I have put the preamble for the file in case it will prompt a suggestion.