settings: Extract code for checking night-mode setting.

We extract the code for checking night mode setting on
the basis of color_scheme value and using matchMedia when
color_scheme is set to automatic. This is currently used
in rendering the correct logo only.

This is a prep commit for fixing loading indicator not
visible on message-edit button in night-mode.

frontend_tests/node_tests/settings_data.js

@@ -225,3 +225,25 @@ test_message_policy(
     "realm_edit_topic_policy",
     settings_data.user_can_edit_topic_of_any_message,
 );
+
+run_test("using_dark_theme", () => {
+    page_params.color_scheme = settings_config.color_scheme_values.night.code;
+    assert.equal(settings_data.using_dark_theme(), true);
+
+    page_params.color_scheme = settings_config.color_scheme_values.automatic.code;
+
+    window.matchMedia = (query) => {
+        assert.equal(query, "(prefers-color-scheme: dark)");
+        return {matches: true};
+    };
+    assert.equal(settings_data.using_dark_theme(), true);
+
+    window.matchMedia = (query) => {
+        assert.equal(query, "(prefers-color-scheme: dark)");
+        return {matches: false};
+    };
+    assert.equal(settings_data.using_dark_theme(), false);
+
+    page_params.color_scheme = settings_config.color_scheme_values.day.code;
+    assert.equal(settings_data.using_dark_theme(), false);
+});

static/js/realm_logo.js

@@ -2,7 +2,7 @@ import $ from "jquery";
 
 import * as channel from "./channel";
 import {page_params} from "./page_params";
-import * as settings_config from "./settings_config";
+import * as settings_data from "./settings_data";
 import * as upload_widget from "./upload_widget";
 
 export function build_realm_logo_widget(upload_function, is_night) {
@@ -82,14 +82,7 @@ export function rerender() {
         );
     }
 
-    if (
+    if (settings_data.using_dark_theme() && page_params.realm_night_logo_source !== "D") {
-        (page_params.color_scheme === settings_config.color_scheme_values.night.code &&
-            page_params.realm_night_logo_source !== "D") ||
-        (page_params.color_scheme === settings_config.color_scheme_values.automatic.code &&
-            page_params.realm_night_logo_source !== "D" &&
-            window.matchMedia &&
-            window.matchMedia("(prefers-color-scheme: dark)").matches)
-    ) {
         $("#realm-logo").attr("src", page_params.realm_night_logo_url);
     } else {
         $("#realm-logo").attr("src", page_params.realm_logo_url);

static/js/settings_data.js

@@ -158,3 +158,18 @@ export function user_can_edit_topic_of_any_message() {
     }
     return user_has_permission(page_params.realm_edit_topic_policy);
 }
+
+export function using_dark_theme() {
+    if (page_params.color_scheme === settings_config.color_scheme_values.night.code) {
+        return true;
+    }
+
+    if (
+        page_params.color_scheme === settings_config.color_scheme_values.automatic.code &&
+        window.matchMedia &&
+        window.matchMedia("(prefers-color-scheme: dark)").matches
+    ) {
+        return true;
+    }
+    return false;
+}