You can use this add-on to resize the top toolbar and the main window: Deck Browser CSS - AnkiWeb
Go to Tools → add-ons → Deck Browser CSS → view files → user_files folder, then:
- To resize the top toolbar, add
#header {zoom: 90%;}to toolbar_injected. - For the deck browser (main window), you can add
body {zoom: 90%;}to deckbrowser_injected.
I think that, when you download the add-on, toolbar_injected is empty while deckbrowser_injected comes with a pre-set custom CSS. You can just delete/disable everything you don’t need.
(Also, if I remember correctly, there is the intention to add some custom styling/theme support to native Anki in the not so distant future)