define mark up for generic sphinx admonitions with a specific title

Here is an example custom.css for overwriting the color of the title and background color of the myownstyle admonition. I used the app.add_stylesheet() call in conf.py.

.rst-content .myownstyle .admonition-title {
   background: #b99976
}

.rst-content .myownstyle {
   background: #e5d3b3
}

To add css files more easily, put them in the _static folder and then add this to conf.py:

def setup(app):
    app.add_stylesheet('custom.css')

If I understood your question correctly, you'd like to apply a custon CSS style to the admonition. You can do this with a :class: attibute.

For example, the following

.. admonition:: my title goes here
   :class: myOwnStyle

   this is the admonition text

renders as

<div class="myownstyle admonition">
  <p class="first admonition-title">my title goes here</p>
  <p class="last">this is the admonition text</p>
</div>

You then add your own style sheet. For example, by a custom layout.html in the _templates directory in your source directory:

{% extends "!layout.html" %}
{% set css_files = css_files + ["_static/customstyle.css"] %}

Then you can play around with CSS styles in your style sheet using a selector for the myownstyle class