Update inline code padding and use border-radius variable

[ndiego]

This is a minor tweak to inline code styles. Previously, the padding was hardcoded and did not adjust based on the size of the font. There are a few instances where inline code is used in headings.

This PR updates the left and right padding to 0.25em. This translates to a padding increase of 1px to 4px for article copy.

While adjusting the padding, I also updated all instances of border-radius: 2px to use the custom variable.

[adamwoodnz]

There are a few instances where inline code is used in headings.

Could you please give an example for testing?

[renintw]

Could you please give an example for testing?

@ndiego icymi.