Add a toggle between dark and light theme

backend/src/translation.rs

@@ -62,6 +62,7 @@ pub enum Sentence {
 
     // Reset password page.
     LostPassword,
+    AskResetChooseNewPassword,
     AskResetButton,
     AskResetAlreadyLoggedInError,
     AskResetEmailAlreadyResetError,
@@ -153,7 +154,7 @@ pub enum Sentence {
 pub const DEFAULT_LANGUAGE_CODE: &str = "en";
 pub const PLACEHOLDER_SUBSTITUTE: &str = "{}";
 
-#[derive(Clone)]
+#[derive(Debug, Clone)]
 pub struct Tr {
     lang: &'static Language,
 }