Make sure you're not using the 'install as userscript' option, because it won't let you change the colour. Use either the 'Stylish' or 'Stylus' browser add-on, and click the 'install style' button near the top of the page.
Click 'advanced style settings' and change the colour there. Then, click 'install style' or 'update style' near the top of the page. Do NOT click 'install as userscript'. Make sure you have either Stylish or Stylus installed.
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If that doesn't work, let me know!