Regarding colours... I think hard-coding colours would be a Bad Idea because everyone has different preferences (should it be dark/dark with a light border or light/light with a light border, and should it be black and white or dark and light shades of grey); and since we (very easily) have the capability to be more flexible, it would be wise to do so. I'd style it with the spoiler block background colour being set to the text colour, ie.:
Text and background colours are set to be different (obviously, as otherwise the text wouldn't be readable), so this way the spoiler block would be easily distinguished from the background and would be consistent with any theme being used.
It could also be done using by setting the text colour to the background colour, though this would make it unclear where the spoiler block is in some cases. A border (say in the text colour, or in another theme colour) could help there. Though my personal opinion would be to set the background colour to the text colour, as above.
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spoiler { background-color: $*color_entry_text; color: $*color_entry_text; }Text and background colours are set to be different (obviously, as otherwise the text wouldn't be readable), so this way the spoiler block would be easily distinguished from the background and would be consistent with any theme being used.
It could also be done using by setting the text colour to the background colour, though this would make it unclear where the spoiler block is in some cases. A border (say in the text colour, or in another theme colour) could help there. Though my personal opinion would be to set the background colour to the text colour, as above.