ok after some research you can't do this.
in vix audioselection.py (the subtitle settings file) you have
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"]elif sub[0] == 1: # teletext[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] menu = [[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] getConfigMenuItem("config.subtitles.ttx_subtitle_colors"),[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] getConfigMenuItem("config.subtitles.ttx_subtitle_original_position"),[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] getConfigMenuItem("config.subtitles.subtitle_fontsize"),[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] getConfigMenuItem("config.subtitles.subtitle_position"),[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] getConfigMenuItem("config.subtitles.subtitle_rewrap"),[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] getConfigMenuItem("config.subtitles.subtitle_borderwidth"),[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] getConfigMenuItem("config.subtitles.showbackground"),[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] getConfigMenuItem("config.subtitles.subtitle_alignment"),[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] getConfigMenuItem("config.subtitles.subtitle_bad_timing_delay"),[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
[TABLE="class: highlight tab-size js-file-line-container"]
[TR]
[TD="class: blob-code blob-code-inner js-file-line"] getConfigMenuItem("config.subtitles.subtitle_noPTSrecordingdelay"),[/TD]
[/TR]
[TR]
[/TR]
[/TABLE]
]
this is defined in esubtitle.cpp
openatv doesn't define subtitles.showbackground in its esubtitle.cpp
The cpp files are C++ files that have to be compiled when your enigma2 build is built. You can't just edit them afterwards.
They don't exist anywhere on your box as such.