Add a dark mode

[richard-cox]

• Allow user to toggle between light and dark modes
• Conditionally show 'OS' mode which honours prefers-color-scheme
  • OS dark mode only triggered on my desktop with FF and about:config ui.systemUsesDarkTheme integer 1
  • Should consider if this is well supported enough to outweigh confusion

[nwmac]

window.getAllSelectors = function() {
var ret = [];
for(var i = 0; i < document.styleSheets.length; i++) {
var rules = document.styleSheets[i].rules || document.styleSheets[i].cssRules;
for(var x in rules) {
if(typeof rules[x].selectorText == 'string') ret.push(rules[x].selectorText);
}
}
return ret;
}

window.haselector = function(selector) {
var selectors = getAllSelectors();
for(var i = 0; i < selectors.length; i++) {
if(selectors[i] == selector) return true;
}
return false;
}

window.haselector('.dark-theme-supported');

[nwmac]

Richard to look at disabling dark theme support if indicated by theme customizer

[nwmac]

LGTM