Hulk have 4 predefined themes which you can see under the src->constants->appConfig.js file:

const AppConfig = {
     …
     …
     lightThemeColors: {
         'primary': '#4d7df2',
         …
         …
         'black':'#000000'
     },
     tealThemeColors: {
         'primary': '#028484',
         …
         …
         'black':'#000000'
     },
     violetThemeColors: {
         'primary': '#53419A',
         …
         …
         'black':'#000000'
     },
     darkThemeColors: {
         'primary': '#4d7df2',
         …
         …
         'black':'#000000'
     }
     …
     …
 }    

If you want to change the theme then you can write light-theme, dark-theme,teal-theme,violet-theme for light , dark, teal and violet respectively & use theme name to override the value of selectedThemeColor in the same file as is given in the below screenshot:

If you want some other theme then you can pass their a class like: dark-theme etc.

That’s all about the Theme’s in the Hulk template.