Each menu item can use own template style in Joomla, SEO is configured in menu items.
MightySites is not related to any GEO.

... phrases. ⋅ Feature: new term setting "Custom Tip Title" to display custom title in term tip. ⋅ Feature: global "Term List" default settings in component options. ⋅ Feature: new term list setting "Term Style" allows to configure preferred output of a term, i.e. with a tooltip. Read full article...

⋅ Change: row repeatable backgrounds moved to "Table Style". ⋅ Feature: "Repeatable Backgrounds" for columns (we already had the same feature but for rows). ⋅ Feature: new frontend/backend advanced settings "Preserve Pagination" and "Preserve Ordering" control the data stored in used session. ⋅ Fix: M ...

... current behavior. Row Number column displays row numbers, you can disable "Row Numbers" option, add "Row Number" column and apply custom position/style easily. More column types are coming... ⋅ Feature: display global component setting value if a report setting is inherited. Read full article...

⋅ Feature: 'Allowed/Disallowed K2 Categories' setting to protect K2 content. ⋅ Feature: new override setting 'Homepage Template Style' to always set custom homepage template style. ⋅ Feature: direct share of native Joomla custom fields (including categories and groups) and/or fields values. ⋅ Feature: ...

Use native or our style and customize them via custom .css file only.

Hi, what's the best way you suggest to apply styles customizations and mantain them through phpbb updates?

I don't think that this will work, each style has different templates and different theme.

Sorry I meant using your style and only change the theme

Our CSS is not enough, certain templates should be adopted as well.