diff --git a/.travis.yml b/.travis.yml
index 72cea2d86c84..d0ff155d4526 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,8 +1,8 @@
 dist: trusty
 
 before_install:
-  - sudo apt-get install coreutils realpath doxygen graphviz python-lesscpy cppcheck coccinelle pcregrep python3-pip
-  - sudo pip3 install flake8
+  - sudo apt-get install coreutils realpath doxygen graphviz cppcheck coccinelle pcregrep python3-pip
+  - sudo pip3 install flake8 lesscpy
 
 script:
   - make static-test
diff --git a/doc/doxygen/Makefile b/doc/doxygen/Makefile
index 92bc60cfa029..e89d64d72dcc 100644
--- a/doc/doxygen/Makefile
+++ b/doc/doxygen/Makefile
@@ -7,14 +7,14 @@ export STRIP_FROM_INC_PATH_LIST=$(shell \
 
 # use lessc (http://lesscss.org/#using-less) for compiling CSS, alternatively
 # fall back to lesscpy (https://github.com/lesscpy/lesscpy)
-ifeq (,$(LESSC))
-  ifneq (,$(shell command -v lessc 2>/dev/null))
-    LESSC=lessc
-  else ifneq (,$(shell command -v lesscpy 2>/dev/null))
-    LESSC=lesscpy
-  endif
+ifneq (,$(shell command -v lessc 2>/dev/null))
+  LESSC_DEFAULT=lessc
+else ifneq (,$(shell command -v lesscpy 2>/dev/null))
+  LESSC_DEFAULT=lesscpy
 endif
 
+LESSC ?= $(LESSC_DEFAULT)
+
 .PHONY: doc
 doc: html
 
@@ -27,14 +27,12 @@ html: src/css/riot.css src/changelog.md
 man: src/changelog.md
 	( cat riot.doxyfile ; echo "GENERATE_MAN = yes" ) | doxygen -
 
-ifneq (,$(LESSC))
 src/css/riot.css: src/css/riot.less src/css/variables.less
 	@$(LESSC) $< $@
 
 src/css/variables.less: src/config.json
 	@grep "^\s*\"@" $< | sed -e 's/^\s*"//g' -e 's/":\s*"/: /g' \
 	  -e 's/",\?$$/;/g' -e 's/\\"/"/g' > $@
-endif
 
 src/changelog.md: src/changelog.md.tmp ../../release-notes.txt
 	@./generate-changelog.py $+ $@
diff --git a/doc/doxygen/src/css/riot.css b/doc/doxygen/src/css/riot.css
index d84883e7c5aa..e6cc9443e72b 100644
--- a/doc/doxygen/src/css/riot.css
+++ b/doc/doxygen/src/css/riot.css
@@ -1,9 +1,3 @@
-/*
- * riot.css
- * Copyright (C) 2017 Freie Universität Berlin
- *
- * Distributed under terms of the LGPLv2.1 license (see LICENSE file).
- */
 table,
 div,
 p,
@@ -18,7 +12,7 @@ dl {
 }
 .navbar-inverse {
   border-radius: 0px;
-  border-bottom: 3px #1a1a1a solid;
+  border-bottom: 3px #1a1a1a  solid;
 }
 .nav-tabs-regs {
   margin-top: 1px;
@@ -53,7 +47,7 @@ dl {
 }
 #MSearchField {
   background: #000000;
-  border: #eeeeee solid 1px;
+  border: #eeeeee  solid 1px;
   color: #ffffff;
   display: inline;
   font: 14px "Helvetica Neue", Helvetica, Arial, sans-serif;
@@ -114,8 +108,8 @@ div.toc-sm {
   background-color: #f5f5f5;
   border: 1px solid #e3e3e3;
   border-radius: 4px;
-  -webkit-box-shadow: inset 0 1px 1px rgba(0, 0, 0, 0.05);
-  box-shadow: inset 0 1px 1px rgba(0, 0, 0, 0.05);
+  -webkit-box-shadow: inset 0 1px 1px rgba(0,0,0,0.05);
+  box-shadow: inset 0 1px 1px rgba(0,0,0,0.05);
 }
 div.toc {
   padding: 19px;
@@ -157,7 +151,7 @@ h3.glow,
 h4.glow,
 h5.glow,
 h6.glow {
-  text-shadow: 0 0 15px #cce6c7;
+  text-shadow: 0 0 15px #cce6c6;
 }
 h2.groupheader {
   border-color: #eeeeee;
@@ -195,7 +189,7 @@ div.fragment {
   display: block;
   font-family: Menlo, Monaco, Consolas, "Courier New", monospace;
   font-size: 14px;
-  line-height: 1.42857143;
+  line-height: 1.428571429;
   margin: 0 0 10px;
   padding: 10px;
   overflow: visible;
@@ -205,8 +199,8 @@ div.fragment {
 }
 div.line.glow {
   background-color: #d8eed8;
-  box-shadow: 0 0 10px #cce6c7;
-  -webkit-box-shadow: 0 0 15px #cce6c7;
+  box-shadow: 0 0 10px #cce6c6;
+  -webkit-box-shadow: 0 0 15px #cce6c6;
 }
 span.lineno {
   background-color: transparent;
@@ -256,8 +250,8 @@ table.directory {
 .memberdecls td.glow,
 .fieldtable tr.glow {
   background-color: #d8eed8;
-  box-shadow: 0 0 15px #cce6c7;
-  -webkit-box-shadow: 0 0 15px #cce6c7;
+  box-shadow: 0 0 15px #cce6c6;
+  -webkit-box-shadow: 0 0 15px #cce6c6;
 }
 .mdescLeft,
 .mdescRight,
@@ -293,8 +287,8 @@ table.directory {
   border-radius: 4px;
 }
 .memitem.glow {
-  box-shadow: 0 0 15px #cce6c7;
-  -webkit-box-shadow: 0 0 15px #cce6c7;
+  box-shadow: 0 0 15px #cce6c6;
+  -webkit-box-shadow: 0 0 15px #cce6c6;
 }
 .memdoc,
 dl.reflist > dd {
@@ -326,7 +320,6 @@ dl.reflist > dt {
   background: #f5f5f5;
   text-shadow: none;
 }
-/* @group Code Colorization */
 a.code {
   color: #3fa687;
 }
@@ -372,8 +365,8 @@ dl.warning {
   border-left: 0px;
   border: 1px solid transparent;
   border-radius: 4px;
-  -webkit-box-shadow: 0 1px 1px rgba(0, 0, 0, 0.05);
-  box-shadow: 0 1px 1px rgba(0, 0, 0, 0.05);
+  -webkit-box-shadow: 0 1px 1px rgba(0,0,0,0.05);
+  box-shadow: 0 1px 1px rgba(0,0,0,0.05);
 }
 dl.attention > dd,
 dl.deprecated > dd,
@@ -404,7 +397,7 @@ dl.warning > dt {
 }
 dl.attention,
 dl.warning {
-  border-color: #f6ccd6;
+  border-color: #f6ccd7;
 }
 dl.attention > dt,
 dl.warning > dt {
@@ -413,7 +406,7 @@ dl.warning > dt {
   border-color: #bd202c;
 }
 dl.bug {
-  border-color: #fce9db;
+  border-color: #fce7db;
 }
 dl.bug a,
 dl.bug a:visited {
@@ -455,17 +448,17 @@ dl.see > dt {
 dl.post,
 dl.pre,
 dl.invariant {
-  border-color: #cce6c7;
+  border-color: #cce6c6;
 }
 dl.post > dt,
 dl.pre > dt,
 dl.invariant > dt {
   color: #5cb85c;
   background-color: #d8eed8;
-  border-color: #cce6c7;
+  border-color: #cce6c6;
 }
 dl.test {
-  border-color: #d2e2f6;
+  border-color: #d2e1f6;
 }
 dl.test a,
 dl.test a:visited {
@@ -477,10 +470,10 @@ dl.test a:hover {
 dl.test > dt {
   color: #5b7ede;
   background-color: #f0f3fc;
-  border-color: #d2e2f6;
+  border-color: #d2e1f6;
 }
 dl.todo {
-  border-color: #d2f4f6;
+  border-color: #d2f3f6;
 }
 dl.todo a,
 dl.todo a:visited {
@@ -492,7 +485,7 @@ dl.todo a:hover {
 dl.todo > dt {
   color: #5bc0de;
   background-color: #f0f9fc;
-  border-color: #d2f4f6;
+  border-color: #d2f3f6;
 }
 #powerTip div {
   font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
@@ -515,4 +508,4 @@ dl.todo > dt {
   border-radius: 4px;
   background-color: #f5f5f5;
   padding: 4px 6px;
-}
+}
\ No newline at end of file