How can I export interface textures? Menu built-in tool allows only to export ships and planets related assets.

There is no built-in way of exporting any other game assets. 

You must extract them yourself from the unity asset files.

I have extracted some assets before if these are the bits your looking for:

CyRob wrote: (Link To Original Post)

Hope this is the stuff your looking for:

ColonizationIcons.zip

Ship Modules.zip

CuriosityTypeIcons.zip

Has to be in parts (4MB limit on attached files)

SystemImprovment Part 1.zip

SystemImprovment Part 2.zip

SystemImprovment Part 3.zip

SystemImprovment Part 4.zip

StarSystemImprovementSacrificeSlaveLarge.png

There is a publicly available tool called Unity Assets Bundle Extractor which can extract the assets, Links to it can be found here: https://github.com/DerPopo/UABE

I did it! Thank you :)