Poor text visibility in custom network warning [Crypto Wallets, Dark Theme]

[ryanml]

Steps to Reproduce

1. Enable dark theme in brave://settings
2. Set up Crypto Wallets, login, head to brave://wallet, navigate to network settings
3. Hit Add Network

Actual result:

Expected result:

Reproduces how often:

100%

[srirambv]

Verification passed on

Brave	1.20.92 Chromium: 88.0.4324.96 (Official Build) beta (64-bit)
Revision	68dba2d8a0b149a1d3afac56fa74648032bcf46b-refs/branch-heads/4324@{#1784}
OS	Linux
Component	1.0.23 Dev

• Verified text is visible when browser theme is set to dark
  

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Verification passed on

Brave	1.20.100 Chromium: 88.0.4324.146 (Official Build) (64-bit)
Revision	406dc88511162d6598242f2c709be1414a042fb0-refs/branch-heads/4324@{#2088}
OS	Windows 10 OS Version 2009 (Build 19042.746)
Component	1.0.23 Dev

• Verified text is visible when browser theme is set to dark
  

---

Verification passed on

Brave	1.22.6 Chromium: 89.0.4389.40 (Official Build) nightly (x86_64)
Revision	2c2ed83cd507b23e4845edd09a7d1dfc727daf4b-refs/branch-heads/4389@{#602}
OS	macOS Version 10.15.7 (Build 19H114)
Component	1.0.23 Dev

• Verified text is visible when browser theme is set to dark