[BUG] Dark Mode button size is too large when clicked

**Describe the bug**
The dark mode button on the nav bar changes size when changed to dark mode. The dark mode button size is large and modifies the size of the nav bar. The size of the icon on click is too large and it is not good looking. I'll fix this if assign this to me.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to 'PetMe website'
2. Click on 'sun icon on the nav bar to see the changes'
3. See error

**Expected behavior**
When the button is presses the icon should only change to a moon icon and the size should be the same and should not modify the size of the nav bar.

**Screenshots**
Bug:

**Desktop (please complete the following information):**
- OS: Windows
- Browser: Chrome, Microsoft Edge