Cabana: Improve contrasts when using dark themes

[projectgus]

Change some hard-coded colors in the Cabana UI that didn't always contrast well when using different system color themes:

• Signal view text/value editor background
• Signal log view heading text
• Binaryview signal box color (when not selected or highlighted)
• Chart title & message text

Before (automatic system theme with dark colors)

Before (manually configured dark theme)

After (automatic system theme with dark colors)

After (manually configured dark theme)

I've only tested this on Linux, but I think it should produce either the same results on systems where the Base colour is White or improved contrast on other systems.