[Desktop] Make Rewards message close button more visible

[eljuno]

Description

Originally reported here https://community.brave.com/t/brave-rewards-wallet-not-connecting-anymore/135056 and here https://community.brave.com/t/brave-rewards-says-my-wallet-is-disconnected-but-also-says-that-its-connected/136328?u=eljuno but have seen similar feedbacks before.

Currently, X close button for Rewards messages is not clearly visible so there's a chance user didn't notice it.

Steps to Reproduce

Waiting for the message to appear by itself? e.g. backup message or connection error message (see screenshot)

Actual result:

Expected result:

More visible

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Version/Channel Information:

• Can you reproduce this issue with the current release? Yes
• Can you reproduce this issue with the beta channel? NDA
• Can you reproduce this issue with the dev channel? NDA
• Can you reproduce this issue with the nightly channel? Yes

Miscellaneous Information:

cc @rebron @srirambv

[GeetaSarvadnya]

Verification passed on

Brave | 1.12.94 Chromium: 84.0.4147.89 (Official Build) dev (64-bit)
-- | --
Revision | 19abfe7bcba9318a0b2a6bc6634a67fc834aa592-refs/branch-heads/4147@{#852}
OS | Windows 10 OS Version 1903 (Build 18362.959)

• Verified the test plan from Increase size of rewards panel alert notification close icon brave-core#6033
• Followed steps from https://bravesoftware.slack.com/archives/C0YL5KMA8/p1588354823102800?thread_ts=1588340348.096000&cid=C0YL5KMA8

Close (x) button in 1.11.x	Close (x) button in 1.12.x