Edit: Advise: This answer is old and a better solution can be found in this same page. This answer referred to MySQL Workbench 6.3 and is outdated. If you are using a new version (8.0 as today) look for @VSingh comment in this very page.
Original answer:
Just a copy of Gaston's answer, but with Monokai theme colors.
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dark-gray: #282828;
brown-gray: #49483E;
gray: #888888;
light-gray: #CCCCCC;
ghost-white: #F8F8F0;
light-ghost-white: #F8F8F2;
yellow: #E6DB74;
blue: #66D9EF;
pink: #F92672;
purple: #AE81FF;
brown: #75715E;
orange: #FD971F;
light-orange: #FFD569;
green: #A6E22E;
sea-green: #529B2F;
-->
<style id="32" fore-color="#DDDDDD" back-color="#282828" bold="No" /> <!-- STYLE_DEFAULT !BACKGROUND! -->
<style id="33" fore-color="#DDDDDD" back-color="#282828" bold="No" /> <!-- STYLE_LINENUMBER -->
<style id= "0" fore-color="#DDDDDD" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_DEFAULT -->
<style id= "1" fore-color="#999999" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_COMMENT -->
<style id= "2" fore-color="#999999" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_COMMENTLINE -->
<style id= "3" fore-color="#DDDDDD" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_VARIABLE -->
<style id= "4" fore-color="#66D9EF" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_SYSTEMVARIABLE -->
<style id= "5" fore-color="#66D9EF" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_KNOWNSYSTEMVARIABLE -->
<style id= "6" fore-color="#AE81FF" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_NUMBER -->
<style id= "7" fore-color="#F92672" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_MAJORKEYWORD -->
<style id= "8" fore-color="#F92672" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_KEYWORD -->
<style id= "9" fore-color="#9B859D" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_DATABASEOBJECT -->
<style id="10" fore-color="#DDDDDD" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_PROCEDUREKEYWORD -->
<style id="11" fore-color="#E6DB74" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_STRING -->
<style id="12" fore-color="#E6DB74" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_SQSTRING -->
<style id="13" fore-color="#E6DB74" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_DQSTRING -->
<style id="14" fore-color="#F92672" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_OPERATOR -->
<style id="15" fore-color="#9B859D" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_FUNCTION -->
<style id="16" fore-color="#DDDDDD" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_IDENTIFIER -->
<style id="17" fore-color="#E6DB74" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_QUOTEDIDENTIFIER -->
<style id="18" fore-color="#529B2F" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_USER1 -->
<style id="19" fore-color="#529B2F" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_USER2 -->
<style id="20" fore-color="#529B2F" back-color="#282828" bold="No" /> <!-- SCE_MYSQL_USER3 -->
<style id="21" fore-color="#66D9EF" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_HIDDENCOMMAND -->
<style id="22" fore-color="#909090" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_PLACEHOLDER -->
<!-- All styles again in their variant in a hidden command -->
<style id="65" fore-color="#999999" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_COMMENT -->
<style id="66" fore-color="#999999" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_COMMENTLINE -->
<style id="67" fore-color="#DDDDDD" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_VARIABLE -->
<style id="68" fore-color="#66D9EF" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_SYSTEMVARIABLE -->
<style id="69" fore-color="#66D9EF" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_KNOWNSYSTEMVARIABLE -->
<style id="70" fore-color="#AE81FF" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_NUMBER -->
<style id="71" fore-color="#F92672" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_MAJORKEYWORD -->
<style id="72" fore-color="#F92672" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_KEYWORD -->
<style id="73" fore-color="#9B859D" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_DATABASEOBJECT -->
<style id="74" fore-color="#DDDDDD" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_PROCEDUREKEYWORD -->
<style id="75" fore-color="#E6DB74" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_STRING -->
<style id="76" fore-color="#E6DB74" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_SQSTRING -->
<style id="77" fore-color="#E6DB74" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_DQSTRING -->
<style id="78" fore-color="#F92672" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_OPERATOR -->
<style id="79" fore-color="#9B859D" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_FUNCTION -->
<style id="80" fore-color="#DDDDDD" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_IDENTIFIER -->
<style id="81" fore-color="#E6DB74" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_QUOTEDIDENTIFIER -->
<style id="82" fore-color="#529B2F" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_USER1 -->
<style id="83" fore-color="#529B2F" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_USER2 -->
<style id="84" fore-color="#529B2F" back-color="#49483E" bold="No" /> <!-- SCE_MYSQL_USER3 -->
<style id="85" fore-color="#66D9EF" back-color="#888888" bold="No" /> <!-- SCE_MYSQL_HIDDENCOMMAND -->
<style id="86" fore-color="#AAAAAA" back-color="#888888" bold="No" /> <!-- SCE_MYSQL_PLACEHOLDER -->