You cannot do it easily - not a feature provided by Microsoft. However there are couple of options for you:
This is probably the best one based on your requirements
Second option, which is not exactly what you are asking for but could still help
Как установить пользовательские цвета для активных и неактивных строк заголовка в стиле Windows 2000 в Windows 8.1?
Я не хочу включать режим высокой контрастности, потому что он делает Internet Explorer непригодным для использования.
You cannot do it easily - not a feature provided by Microsoft. However there are couple of options for you:
This is probably the best one based on your requirements
Second option, which is not exactly what you are asking for but could still help
At the risk of getting under-voted, this is the answer according to my knowledge :
In Windows 8.1, registry options are available only for high contrast themes.
Therefore your only option is to customize a high contrast theme and make it less ugly.
Some third-party tools will help you with that task, for example WindowBlinds ($7.49, trial available).