new hint style

[m13v]

Description

I find bright yellow hint buttons to be too large, and a bit too colorful. I've changed the padding and background color to white. Check out the demo of how it looks like:
https://www.linkedin.com/posts/matthew-diakonov-a84a1911_browsing-through-the-web-with-keyboard-shortcuts-activity-7210104293620686848-EeyU?utm_source=share&utm_medium=member_desktop

Please review the "Which pull requests get merged?" section in CONTRIBUTING.md.

[JUSTIVE]

I like the current style more than this PRs. Those style-related issues may be controversial. But having a theme option and more options would be great.

[UncleSnail]

@m13v Users can change the styles themselves in the settings if they would like (press "?", then "Options" and add your custom CSS to "CSS for Vimium UI"). That is the recommended way to make Vimium look how you would like. It seems to me that there is no need to change the default styling to white with more padding since it is personal preference and the yellow links stand out more. This PR might not fit well with the contribution guidelines and you can consider closing it if you think it appropriate.

[philc]

We'll stick with the current style, but thank you @m13v for posting your implementation for an alternate hint style.