The themes were designed at a time when the customizer was being actively developed and maintained, and standards on the web were much looser.
Unfortunately, in 2020 this no longer appears to be the case. The customizer doesn't recognize modern HTML or CSS and frequently fails without explanation.
Standards have evolved and become much stricter, and previous options that were built in are no longer recommended, such as:
- using custom or Google Fonts (see: System Fonts), causing pagespeed issues
- setting arbitrary font sizes (see: font sizes for food blogs), causing user experience issues
- picking colors without sufficient contrast, causing accessibility issues
In addition, the customizer settings are saved to the theme settings, which means they're lost during updates. This discourages people from performing important updates, even though the previous settings may be outdated, and there could be important safety/optimization updates waiting.
Because of this, we've dropped support for the customizer and instead recommending people don't customize their theme unless they hire a professional, experienced designer.