I've found sort of workaround for this problem.
We can delete new font files from $SPLUNK_HOME/share/splunk/search_mrsparkle/exposed/fonts and keep only Splunk*, droid* and roboto* font files. Splunk will use this fonts after next restart, but in Messages we'll be seeing notification about file modifications.
To remove this notification we can remove lines with deleted font files from *manifest file in your $SPLUNK_HOME folder.
New interface with old fonts:
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