Regarding your first question, it appears that your theme is adding marks to your navigation menu. You’ll need to inspect the element to see what rule is causing it, then override it. (Or choose a different theme.)
Regarding your second question, if your theme has a full-width page template, select it for your BuddyPress pages. If it doesn’t you’ll have to create one, or choose a different theme.
I’m still trying to find what’s going on. I tried link-style none but that doesn’t work.
How exactly can I choose the full-width page template for all of the BuddyPress pages?
@ottiya
You could add a template file named buddypress.php to your theme’s directory, and BP would use that template instead since that file is ahead of page.php in the hierarchy above.
If your theme has full-width template file for page, you can use the same to create buddypress.php
For further details, you can check https://codex.buddypress.org/themes/theme-compatibility-1-7/template-hierarchy/