OK I have mixed icons and widgets for a half hour. I have found some utterly attractive combinations but disabling Built-In Theme ALMOST ALWAYS results in the icons becoming too transparent BUT I get my blue.
Enabling Built-In Theme almost always results in red but the icons have clarity and definition.
Enabling/disabling Dark Theme button an interesting wild card variable but not much.
Conclusions: you have to lighten up your desktop some to make blue work. I don't want it that light.
Can live with the red notifications.
Solved as far as I can see---I'm happy with the looks of my budgie.