Modified Stylish Blockquote by adding blockquote color

Made changes in the `stylish-blockquotes.css` By default, the colour of the blockquote is grey. However, it would look good (or _stylish_ in a literal sense), if the block quote symbol had the same accent colour as that of the theme the user is using. Something like this:  This is a minor change, but felt necessary for to add a _stylish_ element to the stylish blockquote snipet. --- Here is an objective summary of what all changes are made: [+] Added a line of code in the CSS snippet of Stylish Blockquotes (code/css-snippets/stylish-blockquotes.css). [+] Added a preview in the README.md