Thank you!
I will try my best on explaining what I did:
1. I created the SubThemesNames table which I created a relationship with the ThemeNames table.
2. Then I added this criteria in the "Row Source" portion of cboSubTheme :
SELECT tbleSubThemesNames.Theme, tbleSubThemesNames.SubTheme
FROM tbleSubThemesNames
WHERE (((tbleSubThemesNames.Theme)=[Forms]![Form SearchBox]![cboThemeNames]))
ORDER BY tbleSubThemesNames.SubTheme;
3. I'm still able to access my data using the Theme Combobox but the Subtheme combobox in the form is blank. Subtheme is supposed to be derived from the theme list.