Restyle buttons according to updated mockups

[antonok-edm]

The current button designs are outdated.

They should be updated to look like the ones in this mockup:

In dark mode, "Default" style buttons should be styled as follows: white text and #c2c4cf border, Hover style should be #FF7654 bold, weight: 700 text and #FF7654 border. Focused treatment is white text, #FF7654 border with 3px #FF7654 40% alpha border. and pressed state is #FF7654 text and #FF7654 border with 3px #FF7654 40% alpha border

"Primary" style buttons are identical between dark and light themes.

[antonok-edm]

Regarding existing button styles at https://brave.github.io/brave-ui/?path=/story/components--button, as per conversation with @karenkliu:

• "Subtle" style can be removed
• Disabled buttons should be styled the same as enabled, but with 40% opacity
• Primary button style with default gray color can be removed
• "Warning" style can be removed
• "Rewards" button isn't really in line with the design system, but could still be an accent button type for now.