Hi
I guess it's not possible without css, you can try with the following
<dashboard>
<label>chart</label>
<row depends="$hide$">
<panel>
<html>
<style>
#rk g[class="highcharts-axis-labels highcharts-xaxis-labels "] text{
font-weight:bold !important;
}
</style>
</html>
</panel>
</row>
<row>
<panel>
<chart id="rk">
<search>
<query>index=_internal | timechart count by sourcetype</query>
<earliest>-7d@h</earliest>
<latest>now</latest>
</search>
<option name="charting.chart">column</option>
<option name="charting.drilldown">none</option>
</chart>
</panel>
</row>
</dashboard>
