lesson.scss: center images

Pull Request: carpentries/styles#323

diff --combined assets/css/lesson.scss
index 2e1c291e526ff8ae788b081c6f758c6bd763b348,c32d7743fb8768c9420bfdb525732bad97bfb916..d5662e065bfaf5580423c89ad5955d469d2d42ed
--- 1/assets/css/lesson.scss
--- 2/assets/css/lesson.scss
+++ b/assets/css/lesson.scss
@@@ -29,10 -29,10 +29,10 @@@ $color-testimonial: #fc8dc1 !default
  //----------------------------------------
  
  @mixin cdSetup($color) {
 -  color: $color;
 -  border-left: solid 5px $color;
 -  margin-bottom: 0px;
 -  border-radius: 4px 0 0 4px;
 +    color: $color;
 +    border-left: solid 5px $color;
 +    margin: 15px;
 +    border-radius: 4px 0 0 4px;
  }
  
  .error  { @include cdSetup($color-error); }
@@@ -81,11 -81,8 +81,11 @@@ $codeblock-padding: 5px !default
    padding-right: 0;
    border: 1px solid;
    border-color: $color;
 +  border-radius: 4px;
    padding-bottom: $codeblock-padding;
  
 +  margin: 15px;
 +
    h2 {
      padding-top: $codeblock-padding;
      padding-bottom: $codeblock-padding;
@@@ -129,10 -126,6 +129,10 @@@
  font-size: 18px;
  }
  
 +blockquote p {
 +    margin: 5px;
 +}
 +
  //----------------------------------------
  // Override Bootstrap settings.
  //----------------------------------------
@@@ -144,8 -137,10 +144,10 @@@ code 
    background-color: #e7e7e7;
  }
  
- img {
-   max-width: 100%;
+ article img {
+     display: block;
+     margin: 20px auto;
+     max-width: 100%;
  }
  
  //----------------------------------------
@@@ -163,7 -158,7 +165,7 @@@
  footer .copyright,
  footer .help-links
  {
 -    font-size: 18px;
 +    font-size: inherit;
      margin-top: 10px;
      margin-bottom: 10px;
      font-weight: 500;