What are you trying to Style ? 

an example of a modules.config could look like, and exists in the root of your solution 

<?xml version="1.0" encoding="utf-8"?>
<module>
  <assemblies>
    <!-- This adds the Alloy template assembly to the "default module" -->
    <add assembly="Alloy" />
  </assemblies>
  <clientResources>
    <add name="epi-cms.widgets.base" path="~/Resources/Styles.css" resourceType="Style"/>
  </clientResources>
  <dojo>
    <paths>
      <add name="alloy" path="Scripts" />
    </paths>
  </dojo>
</module>

See here for more documentation : module.config (optimizely.com)

If you wanted to change the style of the editor's toolbar, you could add the following configuration to the modules.config file:

    <add name="epi-cms.widget.EditorToolbar">
      <dojoModules>
        <add name="my-custom-style" path="/my/custom/style.css"/>
      </dojoModules>
      <clientResources>
        <add name="my-custom-style" path="/my/custom/style.css"/>
      </clientResources>
    </add>

In this example, we are adding a new dojo module and client resource called "my-custom-style" that references a custom CSS file located at "/my/custom/style.css". This will apply the styles defined in that file to the editor toolbar.

I am trying to get rid some of these spacings between the block-attibutes and display the "hints" under the label instead of a questionmark-icon.

I'm guessing you have seen Scotts post : Restoring Help Text For Description in Latest Optimizely CMS 

Thank you for the suggestions.

When I install a new fresh alloy and add the editorStyle.css and the modules.config files nothing happens.

content of the modules.config

<?xml version="1.0" encoding="utf-8" ?>
<module>
    <clientResources>
        <add name="epi-cms.widgets.base" path="Styles/editorStyles.css" resourceType="Style"/>
    </clientResources>
</module>

Same goes with my other project. Seems to be something missing to get opti load the module.config file.