[GeoffR]

Here is the patch locations of the CSS stylesheet fragment used for the kepub header text if anyone wants to play around with it. Any changes affect the home page header, library header, etc. too.

There seems to be a difference between the Touch and the Glo/Aura/AuraHD, on the Touch the header always uses font-family: Avenir; but on the other models no font-family is set, so whatever is selected fom the font menu is used, or Gothic if Document default is selected.

This example patch forces the Glo/Aura header to use font-family Avenir, the same as the Touch. If you set the footer to match, that would avoid the buggy Gothic/Ryumin fonts from being used at all when Document Default is selected.

firmware 3.3.0:

Spoiler:

<Patch>
patch_name = `Custom header style`
patch_enable = `yes`
#
# Touch? (not tested). Avenir already set.
#replace_string = D8A658, `* {\nfont-size: 16px;\nfont-family: Avenir;\nbackground-color: transparent;\npadding-top:3px;\n}\n\n[qApp_localeName="ja_JP"] {\nfont-family: A-OTF Gothic MB101 Pr6N;\nqproperty-leading: 0;\n}`, `* {\nfont-size: 16px;\nfont-family: Avenir;\nbackground-color: transparent;\npadding-top:3px;\n}\n\n[qApp_localeName="ja_JP"] {\nfont-family: A-OTF Gothic MB101 Pr6N;\nqproperty-leading: 0;\n}`
#
# Glo/Aura. No font specified, set to Avenir.
replace_string = D89484, `* {\n\tbackground-color: transparent;\n\tpadding-top:3px;\n}\n\n[qApp_deviceCodeName="dragon"] {\n\tfont-size: 26px;\n}\n\n[qApp_deviceCodeName="phoenix"] {\n\tfont-size: 21px;\n}\n\n[qApp_localeName="ja_JP"] {\n\tfont-family: A-OTF Gothic MB101 Pr6N;\n\tqproperty-leading: 0;\n}`, `* {\n\tbackground-color: transparent;\n\tpadding-top: 3px;\n\tfont-size: 20px;\nfont-family: Avenir;\n}\n\n[qApp_localeName="ja_JP"] {\n\tfont-family: A-OTF Gothic MB101 Pr6N;\n\tqproperty-leading: 0;\n}`
#
# AuraHD. No font specified, set to Avenir.
#replace_string = D89484, `* {\n\tbackground-color: transparent;\n\tpadding-top:3px;\n}\n\n[qApp_deviceCodeName="dragon"] {\n\tfont-size: 26px;\n}\n\n[qApp_deviceCodeName="phoenix"] {\n\tfont-size: 21px;\n}\n\n[qApp_localeName="ja_JP"] {\n\tfont-family: A-OTF Gothic MB101 Pr6N;\n\tqproperty-leading: 0;\n}`, `* {\n\tbackground-color: transparent;\n\tpadding-top: 3px;\n\tfont-size: 26px;\nfont-family: Avenir;\n}\n\n[qApp_localeName="ja_JP"] {\n\tfont-family: A-OTF Gothic MB101 Pr6N;\n\tqproperty-leading: 0;\n}`
</Patch>

firmware 3.3.1:

Spoiler:

<Patch>
patch_name = `Custom header style`
patch_enable = `yes`
#
# Touch? (not tested). Avenir already set.
replace_string = D8D978, `* {\nfont-size: 16px;\nfont-family: Avenir;\nbackground-color: transparent;\npadding-top:3px;\n}\n\n[qApp_localeName="ja_JP"] {\nfont-family: A-OTF Gothic MB101 Pr6N;\nqproperty-leading: 0;\n}`, `* {\nfont-size: 16px;\nfont-family: Avenir;\nbackground-color: transparent;\npadding-top:3px;\n}\n\n[qApp_localeName="ja_JP"] {\nfont-family: A-OTF Gothic MB101 Pr6N;\nqproperty-leading: 0;\n}`
#
# Glo/Aura. No font specified, set to Avenir.
replace_string = D8C7A4, `* {\n\tbackground-color: transparent;\n\tpadding-top:3px;\n}\n\n[qApp_deviceCodeName="dragon"] {\n\tfont-size: 26px;\n}\n\n[qApp_deviceCodeName="phoenix"] {\n\tfont-size: 21px;\n}\n\n[qApp_localeName="ja_JP"] {\n\tfont-family: A-OTF Gothic MB101 Pr6N;\n\tqproperty-leading: 0;\n}`, `* {\n\tbackground-color: transparent;\n\tpadding-top: 3px;\n\tfont-size: 20px;\nfont-family: Avenir;\n}\n\n[qApp_localeName="ja_JP"] {\n\tfont-family: A-OTF Gothic MB101 Pr6N;\n\tqproperty-leading: 0;\n}`
#
# AuraHD. No font specified, set to Avenir.
#replace_string = D8C7A4, `* {\n\tbackground-color: transparent;\n\tpadding-top:3px;\n}\n\n[qApp_deviceCodeName="dragon"] {\n\tfont-size: 26px;\n}\n\n[qApp_deviceCodeName="phoenix"] {\n\tfont-size: 21px;\n}\n\n[qApp_localeName="ja_JP"] {\n\tfont-family: A-OTF Gothic MB101 Pr6N;\n\tqproperty-leading: 0;\n}`, `* {\n\tbackground-color: transparent;\n\tpadding-top: 3px;\n\tfont-size: 26px;\nfont-family: Avenir;\n}\n\n[qApp_localeName="ja_JP"] {\n\tfont-family: A-OTF Gothic MB101 Pr6N;\n\tqproperty-leading: 0;\n}`
</Patch>

Edit: Note for kpg.exe: Change "replace_string" to "replace_xor_00" and add "\x00" to the end of the replacement string if using kpg.exe.