diff --git a/core/wiki/functions/themes.tid b/core/wiki/functions/themes.tid
index 152ab9b03..8ecbfc97e 100644
--- a/core/wiki/functions/themes.tid
+++ b/core/wiki/functions/themes.tid
@@ -17,9 +17,9 @@ tags: $:/tags/Global
 \function convert.to.in(value) [<value>divide[96]]
 \function convert.to.pc(value) [convert.to.in<value>multiply[6]]
 \function convert.to.pt(value) [convert.to.in<value>multiply[72]]
-\function convert.to.em(value) [<value>divide{$:/themes/tiddlywiki/vanilla/metrics/fontsize}]
+\function convert.to.em(value) [[storyTiddler]is[variable]then<value>divide{$:/themes/tiddlywiki/vanilla/metrics/bodyfontsize}] [[storyTiddler]!is[variable]then<value>divide{$:/themes/tiddlywiki/vanilla/metrics/fontsize}]
 
-\function convert.to.pixels(value,metric) [<metric>match[cm]then<value>multiply[37.8]] [<metric>match[mm]then<value>multiply[37.8]divide[10]] [<metric>match[Q]then<value>multiply[37.8]divide[40]] [<metric>match[in]then<value>multiply[96]] [<metric>match[pc]then<value>multiply[96]divide[6]] [<metric>match[pt]then<value>multiply[96]divide[72]] [<metric>match[em]then<value>multiply{$:/themes/tiddlywiki/vanilla/metrics/fontsize}]
+\function convert.to.pixels(value,metric) [<metric>match[cm]then<value>multiply[37.8]] [<metric>match[mm]then<value>multiply[37.8]divide[10]] [<metric>match[Q]then<value>multiply[37.8]divide[40]] [<metric>match[in]then<value>multiply[96]] [<metric>match[pc]then<value>multiply[96]divide[6]] [<metric>match[pt]then<value>multiply[96]divide[72]] [<metric>match[em]then[storyTiddler]is[variable]then<value>multiply{$:/themes/tiddlywiki/vanilla/metrics/bodyfontsize}] [<metric>match[em]then[storyTiddler]!is[variable]then<value>multiply{$:/themes/tiddlywiki/vanilla/metrics/fontsize}]
 
 \function subtract.one.from.sidebarbreakpoint(value) [is.pixel.value<value>subtract[1]addsuffix[px]] :else[is.cm.value<value>function[convert.to.pixels],<value>,[cm]subtract[1]divide[37.8]addsuffix[cm]] :else[is.mm.value<value>function[convert.to.pixels],<value>,[mm]subtract[1]divide[37.8]multiply[10]addsuffix[mm]] :else[is.Q.value<value>function[convert.to.pixels],<value>,[Q]subtract[1]divide[37.8]multiply[40]addsuffix[Q]] :else[is.in.value<value>function[convert.to.pixels],<value>,[in]subtract[1]divide[96]addsuffix[in]] :else[is.pc.value<value>function[convert.to.pixels],<value>,[pc]subtract[1]divide[96]multiply[6]addsuffix[pc]] :else[is.pt.value<value>function[convert.to.pixels],<value>,[pt]subtract[1]divide[96]multiply[72]addsuffix[pt]] :else[is.em.value<value>function[convert.to.pixels],<value>,[em]subtract[1]divide{$:/themes/tiddlywiki/vanilla/metrics/fontsize}addsuffix[em]]