function updateThemeForStyle(themeName) {
    const selectedTheme = themeOptions[themeName];
    if(selectedTheme === undefined) return;
    const html = document.getElementsByTagName('html')[0];
    Object.keys(selectedTheme).forEach((property, i) => {
        html.style.setProperty(property, selectedTheme[property]);
    });
    colors = getCanvasColors();
}
Updates theme
1) Sets CSS Variables for UI elements
2) Sets color variable for Canvas elements