Fix: wallet icons in dark mode

[katspaugh]

What it solves

Resolves #2374

How this PR fixes it

I've replaced our custom white background on wallet buttons with a semi-transparent white, so that it works for both dark and light icons.

Screenshots

[github-actions]

Branch preview

✅ Deploy successful!

https://fix_onboard_css--walletweb.review-wallet-web.5afe.dev

[github-actions]

ESLint Summary View Full Report

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}

[francovenica]

I see the squares that have the icon inside much darker than in the snapshots.
I tried in chrome, FF and Brave. Also I tried the branch locally and it looks all the same.
It wouldn't be that big of a deal but the Ledger icon is barely visible this way

[katspaugh]

@francovenica I somehow pushed a non-final version, it should be fixed now.

[francovenica]

LGTM now: