Modding the client

[crispy-caesus]

Are there modifications for the client itself? So the user interface and such, besides just custom karts, sounds and tracks.
As an example, there seem to be a bunch of UI elements that I couldn't find in the settings in the attached image (it's from this video: https://www.youtube.com/watch?v=czyonmAYYjo).

[Sven]

You can manually change the icons in data/gui/icons or data/skins if you want to change the icons from the cartoon skin or the menu UI.

[Alayan]

The screenshot here is from the Speedrun Edition, which is a fork of the main code that incorporates changes to the code itself too.

Generally speaking, a great number of changes can be done in the game by editing config files or replacing assets, which includes textures used for UI elements, but some things such as the numerical speed indicator seen in the screenshot require code changes.