Signature Request components - refactor getStyle method

### **Description**

`getStyle` method is duplicated in all 3 signature request components - [MessageSign](https://github.com/MetaMask/metamask-mobile/blob/18dc20b8cc9f3f0707f5eb04f2ac88ee320c93bb/app/components/UI/MessageSign/index.js), [PersonalSign](https://github.com/MetaMask/metamask-mobile/blob/44fd8450a84ebde8510a244ee1eda4b2e12a1df1/app/components/PersonalSign/index.js), [TypedSign](https://github.com/MetaMask/metamask-mobile/blob/18dc20b8cc9f3f0707f5eb04f2ac88ee320c93bb/app/components/UI/TypedSign/index.js)

This function can be extracted out into [ThemeContext](https://github.com/MetaMask/metamask-mobile/blob/bdb7f37c90e4fc923881a07fca38d4e77c73a579/app/util/theme/index.ts).

The function is in-fact duplicated at couple of other places also in code base. Those can also be refactored.

### **Acceptance Criteria**
- Styling of none of the pages refactored is effected.

### **References**
https://docs.google.com/document/d/19xKY4e07UThOa1jsDQA8pjsNLUBRn39EdUbaFpz3OR0/edit?usp=sharing

Start with getStyle in app/components/UI/MessageSign/index.js, app/components/PersonalSign/index.js, and app/components/UI/TypedSign/index.js, then read app/util/theme/index.ts. Compare the duplicated implementations and check the other duplicated locations mentioned in the issue. Done means the shared implementation is used without affecting styling on any refactored page.

Written by the indexing model from the issue text.