Hey @Diyorki,
Definitely, the default theme is better. Just think about default theme like a skeleton. You can customize it. So these themes mostly just add a skin to the default theme. If you want to your site looks like different or have other functions too you need a theme or theme-component. These themes mostly not changes any core feature which can cause seo damage. Hovewer it is possible to cause this kind of issue with simple css styling too but if you notice this kind of issue we can fix it. 
When you activate a theme and something wrong with it you can always switch back to the default theme. You can use themes as separate optional theme next to the default theme. So users can select from different themes but the site default theme is still be the default (light).