Removed hard-coded colors, and use system styles instead, should work better with different themes.

data/stylesheet.css

@@ -51,7 +51,6 @@ padding-left: 5px;
 .openweather-current-databox-captions {
 text-align: right;
 padding-right: 5px;
-color: #999999;
 }
 
 .openweather-forecast-icon {
@@ -70,7 +69,6 @@ min-width: 140px;
 }
 
 .openweather-forecast-day {
-color: #999999;
 font-size: 90%;
 }
 

src/extension.js

@@ -1356,7 +1356,7 @@ const OpenweatherMenuButton = new Lang.Class({
         });
         let rb_captions = new St.BoxLayout({
             vertical: true,
-            style_class: 'openweather-current-databox-captions'
+            style_class: 'popup-status-menu-item openweather-current-databox-captions'
         });
         let rb_values = new St.BoxLayout({
             vertical: true,
@@ -1449,7 +1449,7 @@ const OpenweatherMenuButton = new Lang.Class({
                 style_class: 'openweather-forecast-icon'
             });
             forecastWeather.Day = new St.Label({
-                style_class: 'openweather-forecast-day'
+                style_class: 'popup-status-menu-item openweather-forecast-day'
             });
             forecastWeather.Summary = new St.Label({
                 style_class: 'openweather-forecast-summary'