DCHECK failure - FATAL:theme_properties.cc(142)] Check failed: false.

[simonhong]

STR:

1. Launch browser(dev build) with clean profile
2. Install any custom theme via webstore (ex, https://chrome.google.com/webstore/detail/sea-foam/lahipjfggmgneaopcckkaipmoandaboo)
3. Check DCHECK failure doesn't happen

After merging brave/brave-core#5681, this kind of failure can be reproduced easily.

[66652:66652:0602/203021.851233:FATAL:theme_properties.cc(142)] Check failed: false. This color should only be queried through ThemeService.
#0 0x7f786fe9a7a9 base::debug::CollectStackTrace()
#1 0x7f786fd7df33 base::debug::StackTrace::StackTrace()
#2 0x7f786fd9b6d4 logging::LogMessage::~LogMessage()
#3 0x55c45443d8b9 ThemeProperties::GetDefaultColor()
#4 0x55c454432039 ThemeHelper::GetColor()
#5 0x55c4539e44d5 BookmarkBarInstructionsView::UpdateColors()