Dark mode is not working properly only the theme of the Navbar is changing and the entire website is in light mode.

**Describe the bug**
Dark mode is not working properly. In dark mode, only the theme of the Navbar is changing, the entire webpage is in light mode in both Desktop and Mobile devices. Assign me this issue under GSSOC'23 and allow me to work on a better dark mode for the website.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to 'https://akshitagupta15june.github.io/PetMe/donate.html'
2. Click on 'Change theme icon on the Navbar'
3. See error

**Expected behavior**
In dark mode, the theme of the entire webpage should change.

**Screenshots**

**Desktop**
- OS: Wndows 11
- Browser: Google Chrome
- Version: 113.0.5672.127 (Official Build) (64-bit)

**Smartphone**
- Device: IQOO Neo 6
- OS: Android 13
- Browser: Google Chrome
- Version: 113.0.5672.162

**Additional context**
The dark theme issue persists for the entire website i.e all the webpages.