Can you see a Preferences link at the bottom of the screen on the right? It has a cog next to it ⚙️
About the lightness, there is an option for a light mode as well.
If you can see Preferences:
1. Click Preferences
2. Click Flavours (at left of screen)
3. Click Vanilla Mastodon (at left of screen)
4. Near bottom of screen from the menu labelled "skin" choose the option "Mastodon Light"
5. Click the "Use this" button below that