Tip: Always click Save (Opslaan) after every change in the settings. Otherwise your changes will not be stored.
Go to Settings > General > Profile (Instellingen > Algemeen > Profiel). Here you can change the following:
Profile language (Profieltaal): the language of your screen. This only applies to you. If you set your profile language to English, the company language stays Dutch, for example.
Theme (Thema): switch between dark and light mode.
Phone number (Telefoonnummer): the number used for two-factor authentication (2FA) when you log in.
Password (Wachtwoord): change your password here.
Settings > Profile, with the choice between dark and light theme at the bottom.