Convert application themes to TypeScript

[coderabbitai]

Description

Currently, the application themes are not properly typed with TypeScript, resulting in the use of any type in places like getTheme() in SettingStore.ts.

This issue is to convert the themes to proper TypeScript interfaces/types to improve type safety throughout the application.

Tasks

• Create TypeScript interface(s) for themes
• Convert existing theme definitions to use the new interfaces
• Update references to themes to use proper typing instead of any
• Remove the TODO comment in SettingStore.ts

Related

• Identified in PR: Migrate mobx to zustand #583
• Related comment: Migrate mobx to zustand #583 (comment)

[venraij]

Would it be helpful if i picked this up? I've got some experience and i think this shouldn't be too hard for me to do.