As a User
I want to have a possibility to change the theme between dark and white.

Notes:

• The request body editor (Monaco) already has a builld-in light and dark mode
• As first step implement dark mode as default.
• Add a button where you can change the theme to white
• Take into account that several themes can be added in the future.
• shadcn-ui has an theming option, maybe we can use this.
• If possible, also add here the typography values

Use for dark the variables from figma: https://www.figma.com/file/BOJoxpj7qemqB19bzorKxp/Rest-Client-App?type=design&node-id=95-166&mode=design&t=YDuATL5OrJUNl6KG-4