fix-selection-color.user.js

// ==UserScript==
// @name         fix selection color
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  The selection color is whiteish and I thought it wasn't selecting at all. Oops.
// @author       Josh Parker
// @match        https://www.blunt-therapy.com/*
// @icon         https://www.google.com/s2/favicons?domain=blunt-therapy.com
// @grant        none
// ==/UserScript==

(function fixSelectionColor() {
 const editRules = function editRules(searchTerm, replaceTerm) {
  if (searchTerm.constructor.name !== 'String' || replaceTerm.constructor.name !== 'String') {
   throw new Error('string terms only');
  }

  const styleSheets = [...document.styleSheets];
  styleSheets.forEach((styleSheet, ssi) => {
   try {
    const cssRules = [...styleSheet.cssRules];
    cssRules.forEach((cssRule, cri) => {
     const { cssText } = cssRule;
     if (cssText.includes(searchTerm)) {
      const editedText = cssText.replace(searchTerm, replaceTerm);
      document.styleSheets[ssi].deleteRule(cri);
      document.styleSheets[ssi].insertRule(editedText);
     }
    });
   } catch (e) {
    console.error(e);
    console.log('could not access css rules on styleSheet, ', styleSheet);
   }
  });
 };

 editRules(
  '::selection { background: rgb(251, 251, 255); }',
  '::selection { background: blue; color: white; }',
 );
}());