Update the Onboard wallet modal (Dark mode)

[brunomenezes]

Context

The new brand has new colours and font to be applied. So we could also extend the new look and feel to third-party components we use

PS: Refer to the parent issue for Figma design reference.

Solution

To the possible extent, update the web3-onboard theme colours with the new colours and font rules. We already have some customisation using the exposed CSS variables, so that would be an update to the current one. ui/src/styles/onboard.ts

Onboard custom style docs: https://onboard.blocknative.com/docs/getting-started/custom-styling

Subtasks

• [x] Update existing onboard custom theme definition (Dark mode)

Definition of done

• [x] The onboard modal has the new Dark mode look and feel based on the available colours from Figma designs.
• [x] The onboard modal look and feel match on Staking.
• [x] The onboard modal look and feel match on Rollups.