Fix guidance around modifying theme attributes

[jsoconno]

Describe the bug
The documentation about how to modify themes is outdated and inaccurate.

To Reproduce
Steps to reproduce the behavior:

1. Go to the documentation
2. Read the steps
3. Try to implement them and watch it fail

Expected behavior
You should be able to follow the documentation directly and have it work. We should probably break themes into two sections. One for just the plain vanilla using an exiting theme as is and another that shows how you take modify the Theme object by running from architectures.themes.settings import * and passing settings as an object. For example:

from architecture.themes import DarkMode
from architecture.themes.settings import GraphSettings

graph_settings = GraphSettings(color="red")
theme = DarkMode(graph_settings=graph_settings)

with Graph("Architecture Name", theme = theme):
    pass

Screenshots
None.

Desktop (please complete the following information):
All.

Additional context
None.