If you're finding Mastodon's interface is too dark or too light, you can change its colour scheme to someting that suits your eyes better.

On the website:
1. Click ⚙️ Preferences
2. From the "Site Theme" menu choose Dark, Light or High Contrast
3. Click "Save Changes" at the top right

On the official app:
1. Click the ⚙️ icon to go to settings
2. Choose Dark, Light or Automatic (which uses whatever your phone uses)

On third party apps:
Go to the settings section of your app.