Adjust <kbd> style to mirror markdown code blocks

[cpeel]

During the development of the initial site search, @chrismiceli rightly pointed out that our <kbd> styling sure look a lot like errors being big and red and all. This PR changes the styling to be more reminiscent of common Markdown monospace rendered stylings.

<kbd> styles are mostly used in the quizzes as well as (links to in the sandbox):

• site search
• smoothreading page
• faq/translate.php
• random rules (link is to SA interface)

Sandbox: https://www.pgdp.org/~cpeel/c.branch/update-kbd-style/

A thank you to @srjfoo and @windymilla for their help refining the style.