Standard functions for getting/setting themes

theme.js

            function rememberTheme (theme) {
                // Persist in sessionStorage so theme is applied
                // across pages with file:/// apply remembered theme
                sessionStorage.setItem('mdbook-theme', theme)
                // Persist to localStorage for normal usage as well
                localStorage.setItem('mdbook-theme', theme)
            }

            function getTheme () {
                var theme
                // Get remembered theme from sessionStorage first
                const THEME_SESSION = sessionStorage.getItem('mdbook-theme')
                if (THEME_SESSION) {
                    theme = THEME_SESSION
                } else {
                    // Check localStorage
                    theme = localStorage.getItem('mdbook-theme')
                }
                if (theme) {
                    // Parse out quotes
                    if (theme.startsWith('"') && theme.endsWith('"')) {
                        // Remove quotes from theme
                        theme = theme.slice(1, theme.length - 1)
                        // Persist theme
                        sessionStorage.setItem('mdbook-theme', theme)
                        localStorage.setItem('mdbook-theme', theme)
                    }
                } else {
                    // Use default theme
                    theme = default_theme
                }
                return theme
            }

            function renderTheme () {
                // Look for user specified theme
                try { theme = getTheme() } catch(e) { }
                // Apply theme
                return applyTheme(theme)
            }

            function applyTheme (theme) {
                // Once a theme is changed make sure to persist it
                rememberTheme(theme)
                // Theme is applied via CSS so update the required elements
                document.body.className = theme;
                document.querySelector('html').className = theme + ' js';
            }