Add theme management and settings functionality

- Introduced `ThemeManager` to handle application themes using `ttkthemes`.
- Added `SettingsWindow` for user preferences including theme selection and UI settings.
- Integrated theme selection into the main application with a menu for quick access.
- Enhanced UI components with custom styles based on the selected theme.
- Implemented tooltips for better user guidance across various UI elements.
- Updated dependencies to include `ttkthemes` for improved visual appeal.

requirements.txt

@@ -24,7 +24,9 @@ packaging==25.0
 pandas==2.3.1
     # via -r requirements.in
 pillow==11.3.0
-    # via matplotlib
+    # via
+    #   matplotlib
+    #   ttkthemes
 pyparsing==3.2.3
     # via matplotlib
 python-dateutil==2.9.0.post0
@@ -39,5 +41,7 @@ six==1.17.0
     # via python-dateutil
 tk==0.1.0
     # via -r requirements.in
+ttkthemes==3.2.2
+    # via -r requirements.in
 tzdata==2025.2
     # via pandas