Improve Japanese translation and theme font rendering (#11322)

* Improved Japanese translation

* Update theme.css

* Update theme.css

app/appearance/themes/daylight/theme.css

@@ -175,6 +175,10 @@
     --b3-pdf-dark: #212224;
 }
 
+:lang(ja_JP):root {
+  --b3-font-family: "Helvetica Neue", "Luxi Sans", "DejaVu Sans", "Hiragino Sans", "Yu Gothic UI", sans-serif, "Apple Color Emoji", "Noto Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Android Emoji", "EmojiSymbols";
+}
+
 /* https://github.com/siyuan-note/siyuan/issues/6440 */
 .protyle-action--order:after {
     mix-blend-mode: multiply;

app/appearance/themes/midnight/theme.css

@@ -175,6 +175,10 @@
     --b3-pdf-dark: #212224;
 }
 
+:lang(ja_JP):root {
+  --b3-font-family: "Helvetica Neue", "Luxi Sans", "DejaVu Sans", "Hiragino Sans", "Yu Gothic UI", sans-serif, "Apple Color Emoji", "Noto Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Android Emoji", "EmojiSymbols";
+}
+
 /* https://github.com/siyuan-note/siyuan/issues/6440 */
 .protyle-action--order:after {
     mix-blend-mode: screen;