sed theme-swap for screenshots corrupts the CSS selector too

When rendering a light-theme screenshot of a dark-default page via headless Chrome, do NOT do sed 's/data-theme="dark"/data-theme="light"/' on the whole file. sed processes every line, so besides the <html> attribute it also rewrites the CSS rule :root[data-theme="dark"]{...} into :root[data-theme="light"]{...}, which reassigns the dark palette to the light attribute and the page renders dark anyway. Fix: strip the attribute entirely (sed 's/ data-theme="dark"//') so the base :root (the light/default theme) applies, or target only the html tag. Verify by sampling a bg pixel with PIL, not by eye.