I tried all themes but it looks really bad and not appealing to work with
Mixing a dark desktop theme with a light app theme is unlikely to look very nice.
You could always create a theme to suit your own taste. See: Themes - Audacity Manual
Even with light theme it still looks ugly to the eyes, this is the only app that has this problem, all other apps look nice and follow system theme.
Personally I think it’s far more important that Audacity is the best open source audio editor, and one of the few that is available for Linux. However, this is an open source app, so you should be asking yourself “what are you going to do about it?” (Hint: repeatedly complaining that “others” have not made it look prettier is not constructive and only serves to annoy.)
Background:
Back in 2007, I was annoyed by the poor quality of the pink noise generator in Audacity, so I learned enough C++ to be able to improve it, and submitted a patch to the Audacity team. The patch was accepted and Audacity now has an “instrumentation grade” pink noise generator.
Also:
Audacity can look very nice with a custom theme, for example: Arc-DarK theme (Arc-Dark with DarK icons) - #30 by mischamajskij
In this days the look has big role in how much the app is used, the fall of many great products was their bad looks even if they perform better than others, and the time of ugly linux apps is gone, look at GTK3/4 and Qt apps and see how they look good and pleasing to the eyes.