Make package less theme dependant

[mmcky]

Describe the problem/need and solution

Context

Currently this package is only really compatible with

1. sphinx_book_theme
2. quantecon_book_theme

it would be nice to add css styling to be theme independent

Problem / Idea

Using the sphinx default theme alabaster results in incomplete exercise and solution admonitions missing icons

Solution

Add icons to be theme agnostic through package css(?)

Benefit

More widely useable across various themes.

Guide for implementation

No response

Tasks and updates

No response

[mmcky]

@AakashGfude do you know which css elements I would need to bring in to a sphinx_exercise.css file to support those icons