Use your devtools-theme colors for source views

source-colors.less

// XML source viewer:
#webkit-xml-viewer-source-xml + .header,
#webkit-xml-viewer-source-xml + div + #tree {
  background-color: @background !important;
  color: @foreground !important;
}

#webkit-xml-viewer-source-xml + .header {
  border-bottom: 1px solid @html-comment !important;
  color: @html-comment !important;
  font-family: 'DroidSansMono', Menlo, monospace !important;
  margin: 0 !important;
  padding: 10px 10px 15px 10px !important;
}

#webkit-xml-viewer-source-xml + div + #tree {
  padding-left: 20px !important;
  padding-top: 1em !important;
  margin: 0 !important;
}

// view-source:
body > .webkit-line-gutter-backdrop + table .webkit-line-content {
  background-color: @background !important;
  color: @foreground !important;
}

body > .webkit-line-gutter-backdrop + table .webkit-line-number {
  background-color: @background !important;
  color: @html-comment !important;
  font-size: 0.8125em !important; // = 13px
}

// markup in both views (XML, view-source):
#webkit-xml-viewer-source-xml + div + #tree,
body > .webkit-line-gutter-backdrop + table {
  font-family: 'DroidSansMono', Menlo, monospace !important;
  
  .webkit-html-comment { color: @html-comment !important; } // includes processing instructions
  .webkit-html-doctype { color: @html-comment !important; }
  .webkit-html-tag { color: @html-tag !important; }
  .webkit-html-attribute-name { color: @html-attribute-name !important; }
  .webkit-html-attribute-value { color: @html-attribute-value !important; }
  .webkit-html-attribute-value webkit-html-resource-link { color: @html-link !important; } // anchors
  
  .expand-button,
  .collapse-button {
    -webkit-filter: invert(100%);
  }
  
  ::selection {
    background: @selection !important;
    opacity: 1;
  }
}

// fill the viewport in case the source is shorter:
#webkit-xml-viewer-source-xml + div + #tree::before, 
.webkit-line-gutter-backdrop::before,
body > .webkit-line-gutter-backdrop + table::before {
  background-color: @background !important;
  content: "" !important;
  height: 100% !important;
  left: 0 !important;
  position: absolute !important;
  top: 0 !important;
  width: 100% !important;
  z-index: -1;
}