Feature Request: Use much more noticeable colors instead of grey

## Feature Request Form
### Problem you are trying to address with this feature
Using grey in the buttons and the extension icon in the browser makes it difficult for users with visual challenges

### Your solution
Use much more noticeable colors OR give users the capability to change the color scheme in the buttons or the extension icon

Additional:
The icons in shown in the welcome page are not consistent with what's used in the Hypothesis sidebar.

From user: Jaydax