Some colors in StackEdit were nonsensical for the Solarized dark theme.
Go to Menu -> Settings -> Extensions -> UserCustom Extension. Paste this code in the box provided, then click OK.
userCustom.onReady = function() {
$('head').append('<style>.hljs,.hljs-subst,.hljs-tag .hljs-title,.lisp .hljs-title,.clojure .hljs-built_in,.nginx .hljs-title{color:#eee8d5}.hljs-string,.hljs-title,.hljs-constant,.hljs-parent,.hljs-tag .hljs-value,.hljs-rules .hljs-value,.hljs-rules .hljs-value .hljs-number,.hljs-preprocessor,.hljs-pragma,.haml .hljs-symbol,.ruby .hljs-symbol,.ruby .hljs-symbol .hljs-string,.hljs-aggregate,.hljs-template_tag,.django .hljs-variable,.smalltalk .hljs-class,.hljs-addition,.hljs-flow,.hljs-stream,.bash .hljs-variable,.apache .hljs-tag,.apache .hljs-cbracket,.tex .hljs-command,.tex .hljs-special,.erlang_repl .hljs-function_or_atom,.asciidoc .hljs-header,.markdown .hljs-header,.coffeescript .hljs-attribute{color:#dc322f}pre{display:block;padding:10px;margin:0 0 10.5px;font-size:14px;line-height:1.45;word-break:break-all;word-wrap:break-word;color:#586e75;background-color:#073642;border:1px solid #2aa198;border-radius:4px}.modal-settings .nav-tabs>li>a:hover,.modal-settings .nav-tabs>li>a:focus{color:#93a1a1;border-color:rgba(88,110,117,0.1);background-color:rgba(88,110,117,0.04);border-bottom-color:rgba(0,0,0,0)}</style>');
};