Usually, you don't need this function. Possible theme elements are:
"text", "plot.background", "panel.background", "panel.spacing.y",
"dataline", "datapoint", "statline", "axis.expand.x", "axis.expand.y",
"axis.line.x", "axis.line.y", "axis.ticks.length", "axis.ticks",
"axis.title.y", "axis.title.x", "axis.text.x", "axis.text.y", "plot.title",
"plot.caption", "plot.margin", "casenames", "casenames.strip",
"casenames.background", "casenames.position", "phasenames",
"phasenames.position.x", "separators", "separators.extent", "label.text",
"label.background", "label.padding", "grid", "legend.position",
"legend.background", "legend.text", "legend.title", "legend.margin".
text = c("element_text", "element"),
plot.background = c("element_rect", "element"),
panel.spacing.y = c("simpleUnit", "unit", "unit_v2"),
dataline = "list",
datapoint = "list",
statline = c("element_line", "element" ),
axis.expand.x = "numeric",
axis.expand.y = "numeric",
axis.line.x = c("element_line", "element"),
axis.line.y = c("element_line", "element"),
axis.ticks.length = c("simpleUnit", "unit", "unit_v2"),
axis.ticks = c("element_line", "element"),
axis.title.y = c("element_text", "element"),
axis.title.x = c("element_text", "element"),
axis.text.x = c("element_text", "element"),
axis.text.y = c("element_text", "element"),
plot.title = c("element_text", "element"),
plot.caption = c("element_text", "element"),
plot.margin = c("margin", "simpleUnit", "unit","unit_v2"),
casenames = c("element_text", "element"),
casenames.strip = c("element_rect", "element"),
casenames.background = c("element_rect", "element"),
casenames.position = "character",
phasenames = c("element_text", "element"),
phasenames.position.x = "character",
separators = c("element_line", "element"),
separators.extent = "character",
label.text = c("element_text", "element"),
label.background = c("element_rect", "element" ),
label.padding = "numeric", grid = c("element_line", "element" ),
legend.position = "character",
legend.background = c("element_rect", "element"),
legend.text = c("element_text", "element"),
legend.title = c("element_text", "element"),
legend.margin = c("margin", "simpleUnit", "unit", "unit_v2")