Auto Theme + Wallpaper Switcher (Windows)

This is a lightweight Windows tray app that automatically switches between light and dark mode based on the time of day (6 AM to 6 PM), and optionally changes your desktop wallpaper. It also silently restarts explorer.exe after theme changes to force a full UI refresh (taskbar, Start menu, etc.).

Runs quietly in the background with a tray icon and auto-starts on login (via shell:startup).

Features

Automatically switches between light and dark mode
Optionally changes wallpaper to match the theme
Gracefully skips if wallpaper is missing (no crashes)
Runs silently with a tray icon and no console window
Uses native Windows APIs (registry + wallpaper)
Automatically restarts explorer.exe for full theme refresh

Setup Instructions

Place these files in the same directory:

themechanger.exe       ← compiled binary  
icon.ico               ← tray icon (required)  
light.jpg/png/bmp/...  ← optional day wallpaper  
dark.jpg/png/bmp/...   ← optional night wallpaper

Supported image formats: .jpg, .jpeg, .png, .bmp, .webp

The app will look for light.* and dark.* in its own folder. If no image is found, it simply skips the wallpaper change.

To launch the app automatically when you log in:

Press Win + R
Type: shell:startup and press Enter
Create a shortcut to themechanger.exe:
- Right-click on themechanger.exe.
- Select "Create shortcut".
Move the shortcut you just created into the Startup folder. The Startup folder is the one that opened in step 2.

Done! It will now auto-start silently every time.

No need to edit the registry or install anything!

Build It Yourself

If you'd like to compile from source:

1. Install Go

https://go.dev/dl

2. Initialize your Go module

go mod init themechanger
go mod tidy

3. Build the executable

go build -ldflags="-H windowsgui" -o themechanger.exe

This produces a GUI app (no console window).

Technical Details

Uses Go and the systray library for tray UI
Theme switching via registry keys:
- AppsUseLightTheme
- SystemUsesLightTheme
Wallpaper changes with SystemParametersInfoW API

Explorer restart via:

taskkill /f /im explorer.exe && start explorer.exe

Tray Preview

You must provide your own icon.ico in the same folder — this will be used as the tray icon.

Tip: You can create .ico files from PNGs using online converters like icoconvert.com

Notes

You can manually quit the app from the tray menu
Time check runs every 60 seconds
Changing the clock or timezone will trigger a switch automatically
No admin rights needed
⚠️ Warning: This program will kill and restart explorer.exe to ensure proper theme application after a mode switch. This is necessary because explorer.exe can sometimes become buggy and not fully apply the new theme, leaving behind visual artifacts. Please ensure explorer.exe is not performing any sensitive tasks like copying files before a theme switch occurs, as this process will interrupt those operations.

Optional Ideas

Add GUI to configure start/stop time
Manual override options in tray menu
Geo-based sunrise/sunset switching

License

MIT — free for personal and commercial use.

Made with Go, Windows quirks, and Indocafe Coffeemix ☕

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