Editor width and Blockquotes
There are two annoying features of the default theme. The major one is the incorrect line breaks for code blocks in the editor, which makes editing multiline code blocks to fit into the site without horizontal scroll bars way too cumbersome. The minor one is the nonexistence of a proper output style and a rather disgusting color for blockquotes.
I created a style and made it available as MathematicaStackexchange_Default-Customization (for Firefox and Pale Moon) on https://userstyles.org that removes these.
The width of the editor is extended to make line breaks within code blocks to be matching in the editor and the post, when a vertical scroll bar is present in the editor.
Blockquotes appear to be output-citing by having the same background-color as code and by indenting them. Their top and bottom padding is reduced, while their total width is the same as the default.
Furthermore there is MathematicaStackexchange_Chrome-Customization for Chrome, that additionally increases the with of posts a little bit, to make the code box line width match the most common one.
Example with the style applied:
Example without the style applied: