docs: Add complete THEMES.md gallery with all 13 themes (JhaSourav07#553)

## Description
Creates a complete THEMES.md gallery with preview images, color swatches
for all 13 themes using live commitpulse.vercel.app URLs.

Fixes JhaSourav07#385

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview
N/A

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [ ] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [ ] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

THEMES.md

@@ -1,105 +1,91 @@
-# 🎨 CommitPulse Theme Gallery
+# 🎨 CommitPulse Themes
 
-This gallery showcases all available themes for the CommitPulse streak card.
+All 13 available themes for your CommitPulse badge. Use the `?theme=<slug>` query parameter to apply a
+theme.
 
----
-
-## 🌑 Neon
-
-![Neon](assets/themes/neon.png)
-
-Usage: `/api/streak?user=yourusername&theme=neon`
+```
+https://commitpulse.vercel.app/api/badge?username=YOUR_USERNAME&theme=<slug>
+```
 
 ---
 
-## 🧛 Dracula
-
-![Dracula](assets/themes/dracula.png)
-
-Usage: `/api/streak?user=yourusername&theme=dracula`
+## Theme Gallery
+
+| Theme        | Background | Text      | Accent    |
+| ------------ | ---------- | --------- | --------- |
+| dark         | `#0d1117`  | `#c9d1d9` | `#58a6ff` |
+| light        | `#ffffff`  | `#24292f` | `#0969da` |
+| neon         | `#000000`  | `#00ffcc` | `#ff00ff` |
+| github       | `#0d1117`  | `#ffffff` | `#238636` |
+| dracula      | `#282a36`  | `#f8f8f2` | `#bd93f9` |
+| ocean        | `#0a192f`  | `#ccd6f6` | `#64ffda` |
+| sunset       | `#1a0a0a`  | `#ffd6c0` | `#ff6b35` |
+| forest       | `#0d1f0d`  | `#c8f0c8` | `#39d353` |
+| rose         | `#1f0d14`  | `#f0c8d4` | `#ff6b9d` |
+| nord         | `#2e3440`  | `#d8dee9` | `#88c0d0` |
+| synthwave    | `#0d0221`  | `#f8f8f2` | `#ff2d78` |
+| gruvbox      | `#282828`  | `#ebdbb2` | `#fe8019` |
+| highcontrast | `#0a0a0a`  | `#888888` | `#ff4500` |
 
 ---
 
-## 🌑 Dark
+## Preview URLs
 
-![Dark](assets/themes/dark.png)
+Use these URLs to preview each theme in your README:
 
-Usage: `/api/streak?user=yourusername&theme=dark`
+**dark**
 
----
+![dark](https://commitpulse.vercel.app/api/badge?username=demo&theme=dark)
 
-## 🤍 Light
+**light**
 
-![Light](assets/themes/light.png)
+![light](https://commitpulse.vercel.app/api/badge?username=demo&theme=light)
 
-Usage: `/api/streak?user=yourusername&theme=light`
+**neon**
 
----
+![neon](https://commitpulse.vercel.app/api/badge?username=demo&theme=neon)
 
-## 🐙 GitHub
+**github**
 
-![GitHub](assets/themes/github.png)
+![github](https://commitpulse.vercel.app/api/badge?username=demo&theme=github)
 
-Usage: `/api/streak?user=yourusername&theme=github`
+**dracula**
 
----
+![dracula](https://commitpulse.vercel.app/api/badge?username=demo&theme=dracula)
 
-## 🟠 Gruvbox
+**ocean**
 
-![Gruvbox](assets/themes/gruvbox.png)
+![ocean](https://commitpulse.vercel.app/api/badge?username=demo&theme=ocean)
 
-Usage: `/api/streak?user=yourusername&theme=gruvbox`
+**sunset**
 
----
+![sunset](https://commitpulse.vercel.app/api/badge?username=demo&theme=sunset)
 
-## 🌊 Ocean
+**forest**
 
-![Ocean](assets/themes/ocean.png)
+![forest](https://commitpulse.vercel.app/api/badge?username=demo&theme=forest)
 
-Usage: `/api/streak?user=yourusername&theme=ocean`
+**rose**
 
----
+![rose](https://commitpulse.vercel.app/api/badge?username=demo&theme=rose)
 
-## 🌅 Sunset
+**nord**
 
-![Sunset](assets/themes/sunset.png)
+![nord](https://commitpulse.vercel.app/api/badge?username=demo&theme=nord)
 
-Usage: `/api/streak?user=yourusername&theme=sunset`
+**synthwave**
 
----
+![synthwave](https://commitpulse.vercel.app/api/badge?username=demo&theme=synthwave)
 
-## 🌲 Forest
+**gruvbox**
 
-![Forest](assets/themes/forest.png)
+![gruvbox](https://commitpulse.vercel.app/api/badge?username=demo&theme=gruvbox)
 
-Usage: `/api/streak?user=yourusername&theme=forest`
+**highcontrast**
 
----
-
-## 🌹 Rose
-
-![Rose](assets/themes/rose.png)
-
-Usage: `/api/streak?user=yourusername&theme=rose`
+![highcontrast](https://commitpulse.vercel.app/api/badge?username=demo&theme=highcontrast)
 
 ---
 
-## ❄️ Nord
-
-![Nord](assets/themes/nord.png)
-
-Usage: `/api/streak?user=yourusername&theme=nord`
-
-## 🌆 Synthwave
-
-![Synthwave](assets/themes/synthwave.png)
-
-Retro 80s-inspired palette — deep indigo background, neon pink glow, crisp white text.
-
-Usage: `/api/streak?user=yourusername&theme=synthwave`
-
-## 🔥 High Contrast
-
-Near-black background with vivid red-orange accent and medium-grey text — maximum legibility on both solid and transparent backgrounds for low-vision accessibility.
-
-Usage: `/api/streak?user=yourusername&theme=highcontrast`
+> 💡 **Tip:** Use `?theme=auto` to automatically switch between `light` and `dark` based on the
+> viewer's OS setting.