How do I customize apps and widgets on iPhone Home Screen?

If you want to change your icons, you can follow these steps:

1. Long press on any blank area on your iPhone's home screen. This will cause the icons to start shaking lightly. This will allow you to edit your iPhone's home screen.
2. In the top left-hand corner, where the mini-clock is usually found, click the "Edit" button, then "Customise". This will bring up a small menu at the bottom of your display with 4 Weather icon variants.
3. From here, you can choose what to do. Here is what each option does, you can choose what you prefer:

1. Light: This is the default state for all iOS icons. It will look exactly as it was before iOS 18. Choose this to revert your home screen to it's familiar look.
2. Dark: This is the option you are currently on. It changes nearly all icons to a grey background with a colourful logo.
3. Automatic: This option will automatically shift between light and dark icons, dependant on wether you have dark mode on or off.
4. Tinted: Similarly to dark icons, this option gives your icons a grey background, but instead changes each logo to all be one unified color.

Hope this helps! 😃

This link explains how he did it and how you can adjust it back to the way you'd like them to look --> Customize apps and widgets on the iPhone Home Screen - Apple Support (CA)

Your icons are set to Dark Mode.

Long press a blank portion of the Home Screen — screen enters jiggle mode

Tap edit in upper left corner

Tap customize

At bottom of screen tap light