This is super simple guide how to use this mod.

First let's go through mod options:

• Generate Debug Log - this will generate more messages to output.log file... I suggest to have it disabled and use it only if you run to some problems
  
• Apply styles only for building levels which are included (vanilla behaviour) - This one is quite important! If enabled, styles will work in "vanilla" way. That means when building spawn/upgrades, game will check if style contains assets for given size, zone AND LEVEL - if not, asset is picked from general asset list (all assets).
  If disabled (which I suggest), game will check if style contains asset for given size and zone ONLY. That gives you more control over the district look but then you are also responsible that style contains assets for all levels otherwise buildings won't be able to upgrade. But that can be handled with help of Advanced Building Level Control mod. That is really useful mod to have.
  
• Show standalone District Styles Editor button (UUI button is always shown) - If enabled, standalone button for DSE will be shown along UUI button. I suggest to disable. I actually plan to remove that button completely.
  
• Options for import are described in guide for importing Building Themes.

Now, how to use mod... if you click "DS Edit" button from UUI (or standalone DSPE button), you get District Style Editor:

UI1 

Community Created Artwork for Steam Artwork

By: meda22

FILTER (red)
This will allow you filter listed buildings (middle column) by zone, source (in-game or steam - custom), included/excluded in selected style, level, size, height range and name.
If you click on specific zone, it will select / deselect that zone. If you double-click on zone, it will select ONLY that zone and deselect others. Button ALL will select all zones. Button NONE will de-select all zones.
You can also chose how should be filtered buildings sorted.

LIST OF STYLES (green)
Here you will see all enabled styles. You can create new style or delete some existing style which is not built-in (then delete button will be disabled). Built-in styles can't be modified neither (so you can't add/remove buildings in them).
You can select style for edit by clicking on it (it will get blue background).

LIST OF BUILDINGS (blue)
When you selected style, you can see which buildings are included (checked checkbox) or excluded (unchecked checkbox). You can add/remove single building by clicking on checkbox.
You can also add/remove whole list of currently filtered buildings by using buttons under the building list (yellow rectangle on screenshot), to include all of them to style or none of them to style.

Any change in building list of a style is immediately saved, so be careful.

You can assign style to areas without any district in you city - which is useful especially when you start new game and you don't have ability to create a district yet.

UI2 

Community Created Artwork for Steam Artwork

By: meda22

How to do it? Open city information panel (panel where you can rename your city), click button styles, check styles which you want to use and click SAVE to save your choices otherwise styles won't be assigned.

UI3 

Community Created Artwork for Steam Artwork

By: meda22

How to do it? Open district information panel, click button styles, check styles which you want to use and click SAVE to save your choices otherwise styles won't be assigned and you will be disappointed.